ORIGIN OF METEORS--THE NEBULAR HYPOTHESIS.
In regard to the physical history of those meteoric masses which,
in such infinite numbers, traverse the interplanetary spaces, our
knowledge is exceedingly limited. Such as have reached the earth's
surface consist of various elements in a state of combination. It
has been remarked, however, by a distinguished scientist[32] that
"the character of the constituent particles of meteorites, and their
general microscopical structure, differ so much from what is seen
in terrestrial volcanic rocks, that it appears extremely improbable
that they were ever portions of the moon, or of a planet, which
differed from a large meteorite in having been the seat of a more or
less modified volcanic action." As the celebrated nebular hypothesis
seems to afford a very probable explanation of the origin of those
bodies, whether in the form of rings or sporadic masses, its brief
consideration may not be destitute of interest. We will merely premise
that the existence of true nebulæ in the heavens--that is, of matter
consisting of luminous gas--has been placed beyond doubt by the
revelations of the spectroscope.
As a group, our solar system is comparatively isolated in space; the
distance of the nearest fixed star being at least seven thousand times
that of Neptune, the most remote known planet. Besides the central or
controlling orb, it contains, so far as known at present, ninety-nine
primary planets, eighteen satellites, three planetary rings, and nearly
eight hundred comets. In taking the most cursory view of this system we
cannot fail to notice the following interesting facts in regard to the
motions of its various members:
1. The sun rotates on his axis from west to east.
2. The primary planets all move nearly in the plane of the sun's
equator.
3. The orbital motions of all the planets, primary and secondary,
except the satellites of Uranus and Neptune, are in the same
_direction_ with the sun's rotation.
4. The direction of the rotary motions of all the planets, primary and
secondary, in so far as has been observed, is identical with that of
their orbital revolutions; viz., from west to east.
5. The rings of Saturn revolve about the planet in the same direction.
6. The planetary orbits are all nearly circular.
7. The _cometary_ is distinguished from the _planetary_ portion of the
system by several striking characteristics: the orbits of comets are
very eccentric and inclined to each other, and to the ecliptic at all
possible angles. The motions of a large proportion of comets are _from
east to west_. The physical constitution of the latter class of bodies
is also very different from that of the former; the matter of which
comets are composed being so exceedingly attenuated, at least in some
instances, that fixed stars have been distinctly visible through what
appeared to be the densest portion of their substance.
None of these facts are accounted for by the law of gravitation. The
sun's attraction can have no influence whatever in determining either
the _direction_ of a planet's motion, or the eccentricity of its orbit.
In other words, this power would sustain a planetary body moving from
east to west, as well as from west to east; in an orbit having any
possible degree of inclination to the plane of the sun's equator, no
less than in one coincident with it; or, in a very eccentric ellipse,
as well as in one differing but little from a circle. The consideration
of the coincidences which we have enumerated led Laplace to conclude
that their explanation must be referred to the _mode_ of our system's
formation--a conclusion which he regarded as strongly confirmed by
the contemporary researches of Sir William Herschel. Of the numerous
nebulæ discovered and described by that eminent observer, a large
proportion could not, even by his powerful telescope, be resolved into
stars. In regard to many of these, it was not doubted that glasses
of superior power would show them to be extremely remote sidereal
clusters. On the other hand, a considerable number were examined which
gave no indications of resolvability. These were supposed to consist
of self-luminous, nebulous matter--the chaotic elements of future
stars. The great number of these irresolvable nebulæ scattered over
the heavens and apparently indicating the various stages of central
condensation, very naturally suggested the idea that the solar system,
and perhaps every other system in the universe, originally existed
in a similar state. The sun was supposed by Laplace to have been an
exceedingly diffused, rotating nebula, of spherical or spheroidal
form, extending beyond the orbit of the most distant planet; the
planets as yet having no separate existence. This immense sphere of
vapor, in consequence of the radiation of heat and the continual
action of gravity, became gradually more dense, which condensation was
necessarily attended by an increased angular velocity of rotation. At
length a point was thus reached where the centrifugal force of the
equatorial parts was equal to the central attraction. The condensation
of the interior meanwhile continuing, the equatorial zone was detached,
but necessarily continued to revolve around the central mass with
the same velocity that it had at the epoch of its separation. If
perfectly uniform throughout its entire circumference, which would be
highly improbable, it would continue its motion in an unbroken ring,
like that of Saturn; if not, it would probably collect into several
masses, having orbits nearly identical. "These masses should assume a
spheroidal form, with a rotary motion in the direction of that of their
revolution, because their inferior articles have a less real velocity
than the superior; they have therefore constituted so many planets in a
state of vapor. But if one of them was sufficiently powerful to unite
successively by its attraction all the others about its center, the
ring of vapors would be changed into one spheroidal mass, circulating
about the sun, with a motion of rotation in the same direction with
that of revolution."[33] Such, according to the theory of Laplace, is
the history of the formation of the most remote planet of our system.
That of every other, both primary and secondary, would be precisely
similar.
In support of the nebular hypothesis, of which the foregoing is a
brief general statement, we remark that _it furnishes a very simple
explanation of the motions and arrangements of the planetary system_.
In the first place, it is evident that the separation of a ring would
take place at the equator of the revolving mass, where of course the
centrifugal force would be greatest. These concentric rings--and
consequently the resulting planets--would all revolve _in nearly the
same plane_. It is evident also that the central body must have a
revolution on its axis _in the same direction with the progressive
motion of the planets_. Again: at the breaking up of a ring, the
particles of nebulous matter more distant from the sun would have
a greater absolute velocity than those nearer to it, which would
produce the observed _unity of direction in the rotary and orbital
revolutions_. The motions of the satellites are explained in like
manner. The hypothesis, moreover, accounts satisfactorily for the fact
that the orbits of the planets are all nearly circular. And finally,
it presents an obvious explanation of the rings of Saturn. It would
almost seem, indeed, as if these wonderful annuli had been left by the
Architect of Nature, as an index to the creative process.
The argument derived from the motions of the various members of the
solar system is not new, having been forcibly stated by Laplace,
Pontécoulant, Nichol, and other astronomers. Its full weight and
importance, however, have not, we think, been duly appreciated. That a
common physical cause has determined these motions, must be admitted
by every philosophic mind. But apart from the nebular hypothesis,
no such cause, adequate both in mode and measure, has ever been
suggested;--indeed none, it seems to us, is conceivable. The phenomena
which we have enumerated _demand_ an explanation, and this demand
is met by the nebular hypothesis. It will be found, therefore, when
closely examined, that the evidence afforded by the celestial motions
is sufficient to give the theory of Laplace a very high degree of
probability.
A comparison of the facts known in regard to comets, falling-stars,
and meteoric stones, seems to warrant the inference that they are
bodies of the same nature, and perhaps of similar origin; differing
from each other mainly in the accidents of magnitude and density. The
hypothesis of Laplace very obviously accounts for the formation of
planets and satellites, moving in the same direction, and in orbits
nearly circular; but how, it may be asked, can the same theory explain
the extremely eccentric, and in some cases retrograde, motions of
comets and aerolites? This is the question to which we now direct our
attention.
After the nuclei of the solar and sidereal systems had been established
in the primitive nebula, and when, in consequence, immense gaseous
spheroids had collected around such nuclei, we may suppose that about
the points of equal attraction between the sun and neighboring
systems, portions of nebulous matter would be left in equilibrio.
Such outstanding nebulosities would gradually contract through the
operation of gravity; and if, as would sometimes be the case, the solar
attraction should preponderate, they would commence falling toward our
system. Unless disturbed by the planets they would probably move round
the sun in parabolas. Should they pass, however, near any of the large
bodies of the system, their orbits might be changed into ellipses by
planetary perturbation. Such was the view of Laplace in regard to the
origin of comets.
It seems probable, however, that many of these bodies originated
_within_ the solar system, and belong properly to it. The outer rings
thrown off by the planets may have been at too great distances from the
primaries to form stable satellites. Such masses would be separated
by perturbation from their respective primaries, and would revolve
round the sun in independent orbits. Again: small portions of nebulous
matter may have been abandoned as primary rings, at various intervals
between the planetary orbits. At particular distances such rings would
be liable to extraordinary perturbations, in consequence of which
their orbits would ultimately assume an extremely elliptical form,
like those of comets, and perhaps also those of meteors. It was shown
in Chapter XIII. that several such regions occur in the asteroid zone
between Mars and Jupiter. We may add, in confirmation of this view,
that there are twelve known comets whose periods are included between
those of Flora and Jupiter. Their motions are all direct; their orbits
are less eccentric than those of other comets; and the mean of their
inclinations is about the same as that of the asteroids. These facts
certainly appear to indicate some original connection between these
bodies and the zone of minor planets.
The nebular hypothesis, it is thus seen, accounts satisfactorily for
the origin of comets, aerolites, fire-balls, shooting-stars, and
meteoric rings; regarding them all as bodies of the same nature, moving
in cometary orbits about the sun. In this theory, the zodiacal light is
an immense swarm of meteor-asteroids; so that the meteoric theory of
solar heat, explained in a previous chapter, finds its place as a part
of the same hypothesis.
CONCLUSION.
Some of the prominent results of observation and research in meteoric
astronomy may be summed up as follows:
1. The shooting-stars of November, August, and other less noted epochs,
are derived from elliptic rings of meteoric matter which intersect the
earth's orbit.
2. Meteoric stones and the matter of shooting-stars coexist in the
same rings; the former being merely collections or aggregations of the
latter.
3. The most probable period of the November meteors is thirty-three
years and three months. Leverrier's elements of this ring agree so
closely with Oppolzer's elements of the comet of 1866 as to render it
probable that the latter is merely _a large meteor_ belonging to the
same annulus.
4. The spectroscopic examination of this comet (of 1866) by William
Huggins, F.R.S., indicated that the nucleus was self-luminous, that
the coma was rendered visible by reflecting solar light, and that "the
material of the comet was similar to the matter of which the gaseous
nebulæ consist."
5. The time of revolution of the August meteors is believed to be about
105 years. M. Schiaparelli has found a striking similarity between the
elements of this ring and those of the third comet of 1862. The same
distinguished astronomer has shown, moreover, that a nebulous mass of
considerable extent, drawn into the solar system _ab extra_, would form
a _ring_ or _stream_.
6. The aerolitic epochs, established with more or less certainty, are
the following:
1. February 15th-19th.
2. March 12th-15th.
3. April 10th-12th.
4. April 18th-26th.
5. May 8th-14th; or especially, 12th-13th.
6. May 19th.
7. July 13th-14th.
8. July 26th.
9. August 7th-11th.
10. October 13th-14th.
11. November 11th-14th.
12. November 27th-30th.
13. December 7th-13th.
About one-half of this number are also known as shooting-star epochs.
7. The epoch of November 27th-30th corresponds with that of the earth's
crossing the orbit of Biela's two comets. The aerolites of this epoch
may therefore have been moving in nearly the same path.
8. A greater number of aerolitic falls are observed--
1. By day than by night.
2. In the afternoon than in the forenoon.
3. When the earth is in aphelion than when in perihelion.
The first fact is accounted for by the difference in the number of
observers; the second indicates that a majority of aerolites have
direct motion; and the third is dependent on the relative lengths of
the day and night in the aphelic and perihelic portions of the orbit.
9. The observed velocities of meteorites are incompatible with the
theory of their lunar origin.
10. If the meteoric swarm of November 14th has a period of thirty-three
years, Biela's comet passed very _near_, if not actually _through_ it
toward the close of 1845, about the time of the comet's separation. Was
the division of the cometary mass produced by the encounter?
11. The rings of Saturn may be regarded as dense meteoric masses, and
the principal or permanent division accounted for by the disturbing
influence of the interior satellites.
12. The asteroidal space between Mars and Jupiter is probably a wide
meteoric ring in which the largest aggregations are visible as minor
planets. In the distribution of the mean distances of the known members
of the group a clustering tendency is quite obvious.
13. The meteoric masses encountered by Encke's comet may account for
the shortening of the period of the latter without the hypothesis of an
ethereal medium.
APPENDIX.
A.
The Meteors of November 14th.
The _American Journal of Science and Arts_ for May, 1867 (received by
the author after the first chapters of this work had gone to press),
contains an interesting article by Professor Newton "On certain recent
contributions to Astro-Meteorology." Of the five possible periods of
the November ring, first designated by Professor N, it is now granted
that the longest, viz., 33-1/4 years, is most probably the true one.
The results of Leverrier's researches in regard to the epoch at which
this meteoric mass was introduced into the solar system, are given in
the same article. This distinguished astronomer supposes the group of
meteors to have been thrown into an elliptic orbit by the disturbing
influence of Uranus. The meteoric stream, according to the most
trustworthy elements of its orbit, passed extremely near that planet
about the year 126 of our era; which date is therefore assigned by
Leverrier as the probable time of its entrance into the planetary
system. This result, however, requires confirmation.
Although the earliest display of the November meteors, so far as
certainly known, was that of the year 902, several more ancient
exhibitions may, with some probability, be referred to the same epoch.
These are the phenomena of 532, 599, and 600, A.D., and 1768, B.C. (See
Quetelet's Catalogue.) The time of the year at which these showers
occurred is not given. The _years_, however, correspond very well
with the epochs of the maximum display of the November meteors. The
intervals arranged in consecutive order, are as follows:
From B.C. 1768 to A.D. 532, 69 periods of 33·319 years each.
" A.D. 532 to " 599·5, 2 " 33·750 "
" " 599·5 to " 902, 9 " 33·614 "
" " 902 to " 934, 1 " 32·000 "
" " 934 to " 1002, 2 " 34·000 "
" " 1002 to " 1101, 3 " 33·000 "
" " 1101 to " 1202, 3 " 33·667 "
" " 1202 to " 1366, 5 " 32·800 "
" " 1366 to " 1533, 5 " 33·400 "
" " 1533 to " 1698, 5 " 33·000 "
" " 1698 to " 1799, 3 " 33·667 "
" " 1799 to " 1833, 1 " 34·000 "
" " 1833 to " 1866, 1 " 33·000 "
The first three dates are alone doubtful. The whole number of intervals
from B.C. 1768 to A.D. 1866 is 109, and the mean length is 33·33 years.
The perturbations of the ring by Jupiter, Saturn, and Uranus, are
doubtless considerable. It is worthy of note that--
14 periods of Jupiter are nearly equal to 5 of the ring.
9 " Saturn " " 8 "
23 " Uranus " " 58 "
This group or stream has its perihelion at the orbit of the earth; its
aphelion, at that of Uranus. (See diagram, p. 24.) It must therefore
produce star-showers at the latter as well as at the former. Our
planet, moreover, at each encounter appropriates a portion of the
meteoric matter; while at the remote apsis of the stream Uranus in all
probability does the same. The matter of the ring will thus by slow
degrees be gathered up by the two planets.
B.
Comets and Meteors.
The recent researches and speculations of European astronomers in
regard to the origin of comets and of meteoric streams, have suggested
to the author the propriety of reproducing the following extracts from
an article written by himself, in July, 1861, and published in the
_Danville Quarterly Review_ for December of that year:
"Different views are entertained by astronomers in regard to the
_origin_ of comets; some believing them to enter the solar system _ab
extra_; others supposing them to have originated within its limits. The
former is the hypothesis of Laplace, and is regarded with favor by many
eminent astronomers. It seems to afford a plausible explanation of the
paucity of large comets during certain long intervals of time. In one
hundred and fifty years, from 1600 to 1750, sixteen comets were visible
to the naked eye; of which eight appeared in the twenty-five years from
1664 to 1689. Again, during sixty years from 1750 to 1810, only five
comets were visible to the naked eye, while in the next fifty years
there were double that number. Now, according to Laplace's hypothesis,
patches of nebulous matter have been left nearly in equilibrium in the
interstellar spaces. As the sun, in his progressive motion, approaches
such clusters, they must, by virtue of his attraction, move toward
the center of our system; the nearer portions with greater velocity
than the more remote. The nebulous fragments thus introduced into our
system would constitute comets; those of the same cluster would enter
the solar domain at periods not very distant from each other; the forms
of their orbits depending upon their original relative positions with
reference to the sun's course, and also on planetary perturbations. On
the other hand, the passage of the system through a region of space
destitute of this chaotic vapor would be followed by a corresponding
paucity of comets.
"Before the invention of the telescope, the appearance of a comet was
a comparatively rare occurrence. The whole number visible to the naked
eye during the last three hundred and sixty years has been fifty-five;
or a mean of fifteen per century. The recent rate of telescopic
discovery, however, has been about four or five annually. As many of
these are extremely faint, it seems probable that an indefinite number,
too small for detection, may be constantly traversing the solar domain.
If we adopt Laplace's hypothesis of the origin of comets, we may
suppose an almost continuous fall of primitive nebular matter toward
the center of the system--the _drops_ of which, penetrating the earth's
atmosphere, produce _sporadic_ meteors; the larger aggregations forming
comets. The disturbing influence of the planets may have transformed
the original orbits of many of the former, as well as of the latter,
into ellipses. It is an interesting fact that the motions of some
luminous meteors--or _cometoids_, as perhaps they might be called--have
been decidedly indicative of an origin beyond the limits of the
planetary system.
"But how are the phenomena of _periodic_ meteors to be accounted for,
in accordance with this theory?
"The division of Biela's comet into two distinct parts suggests several
interesting questions in cometary physics. The nature of the separating
force remains to be discovered; 'but it is impossible to doubt that it
arose from the divellent action of the sun, whatever may have been the
mode of operation.'
"'A signal manifestation of the influence of the sun,' says a
distinguished writer, 'is sometimes afforded by the breaking up
of a comet into two or more separate parts, on the occasion of its
approach to the perihelion. Seneca relates that Ephoras, an ancient
Greek author, makes mention of a comet which before vanishing was
seen to divide itself into two distinct bodies. The Roman philosopher
appears to doubt the possibility of such a fact; but Keppler, with
characteristic sagacity, has remarked that its actual occurrence was
exceedingly probable. The latter astronomer further remarked that there
were some grounds for supposing that two comets, which appeared in the
same region of the heavens in the year 1618, were the fragments of a
comet that had experienced a similar dissolution. Hevelius states that
Cysatus perceived in the head of the great comet of 1618 unequivocal
symptoms of a breaking up of the body into distinct fragments. The
comet when first seen in the month of November, appeared like a round
mass of concentrated light. On the 8th of December it seemed to be
divided into several parts. On the 20th of the same month it resembled
a multitude of small stars. Hevelius states that he himself witnessed a
similar appearance in the head of the comet of 1661.'[34] Edward Biot,
moreover, in his researches among the Chinese records, found an account
of 'three dome-formed comets' that were visible simultaneously in 896,
and pursued very nearly the same apparent path.
"Another instance of a similar phenomenon is recorded by Dion Cassius,
who states that a comet which appeared eleven years before our era,
separated itself into several small comets.
"These various examples are presented at one view, as follows:
"I. Ancient bipartition of a comet.--_Seneca, Quæst. Nat._,
_lib. VII. cap. XVI._
"II. Separation of a comet into a number of fragments, 11
B.C.--_Dion Cassius._
"III. Three comets seen simultaneously pursuing the same orbit,
A.D. 896--_Chinese records--Comptes Rendus_, tom. xx. 1845, p.
334.
"IV. Probable separation of a comet into parts, A.D.
1618.--_Hevelius_, _Cometographia_, p. 341.--_Keppler_, _De
Cometis_, p. 50.
"V. Indications of separation, 1661.--_Hevelius_,
_Cometographia_, p. 417.
"VI. Bipartition of Biela's comet, 1845-6.
"In view of these facts it seems highly probable, if not absolutely
certain, that the process of division has taken place in several
instances besides that of Biela's comet. May not the force, whatever
it is, that has produced _one_ separation, again divide the parts? And
may not this action continue until the fragments become invisible?
According to the theory now generally received, the periodic phenomena
of shooting-stars are produced by the intersections of the orbits of
such nebulous bodies with the earth's annual path. Now there is reason
to believe that these meteoric rings are very elliptical, and in this
respect wholly dissimilar to the rings of primitive vapor which,
according to the nebular hypothesis, were successively abandoned at
the solar equator; in other words, that the matter of which they are
composed moves in _cometary_ rather than _planetary_ orbits. May not
our periodic meteors be the _debris_ of ancient but now disintegrated
comets, whose matter has become distributed around their orbits?"
C.
Biela's Comet and the Meteors of November 27th-30th.
At the close of Chapter IV. it was suggested that the meteors of
November 27th-30th might possibly be derived from a ring of meteoric
matter moving in the orbit of Biela's comet. Since that chapter was
written similar conjectures have been started in the _Astronomische
Nachrichten_[35] by Dr. Edmund Weiss and Prof. d'Arrest. The latter
attempts to show that the December meteors may be derived from the same
ring. The question will doubtless be decided at no distant day.
D.
The First Comet of 1861 and the Meteors of April 20th.
Recent investigations render it probable that the orbit of the first
comet of 1861 is identical with that of the meteors of April 20th. The
orbit is nearly perpendicular to the ecliptic.
FOOTNOTES:
[1] For a full description, see Silliman's Journal for January and
April, 1834 (Prof. Olmsted's article). Also a valuable paper, in the
July No. of the same year, by Prof. Twining.
[2] Physique du Globe, Chap. IV.
[3] Professor Olmsted estimated the number of meteors, visible at New
Haven, during the night of November 12th-13th, 1833, at 240,000.
[4] Conde says, "there were seen, as it were lances, an infinite number
of stars, which scattered themselves like rain to the right and left,
and that year was called 'the year of stars.'"
[5] In 1202, "on the last day of Muharrem, stars shot hither and
thither in the heavens, eastward and westward, and flew against
one another like a scattering swarm of locusts, to the right and
left; this phenomenon lasted until daybreak; people were thrown
into consternation, and cried to God the Most High with confused
clamor."--Quoted by Prof. Newton, in Silliman's Journal, May, 1864.
[6] Am. Journ. of Sci. and Arts, May and July, 1864.
[7] The stream or arc of meteors is several years in passing its node.
The first indication of the approach of the display of 1866 was the
appearance of meteors in unusual numbers at Malta, on the 13th of
November, 1864. The great length of the arc is indicated, moreover, by
the showers of 931 and 934.
[8] Silliman's Journ. for Sept. and Nov., 1861.
[9] The numerical results here given are those found by Professor
Newton. See Silliman's Journ. for March, 1865.
[10] The diameters of the asteroids are derived from a table by Prof.
Lespiault, in the Rep. of the Smithsonian Inst. for 1861, p. 216.
[11] "It appears probable, from the researches of Schreibers, that 700
fall annually."--Cosmos, vol. i. p. 119 (Bohn's Ed.). Reichenbach makes
the number much greater.
[12] New Concord is close to the Guernsey County line. Nearly all the
stones fell in Guernsey.
[13] Cosmos, vol. i. p. 120.
[14] Leverrier's Annals of the Observatory of Paris, vol. i. p. 38.
[15] "This is a remarkable example of a stone arriving on the earth
with a temperature approaching that of the interplanetary spaces.
Aerolites containing much iron, a substance which conducts heat well,
get thoroughly heated by their passage through the atmosphere. But the
stony aerolites, containing less iron, conducting heat badly, preserve
in their interior the temperature of the locality from which they fall;
their surface only is heated, and generally fused. When the stones are
large, the _excessive cold_ of their interior portion, which must be
nearly that of interplanetary space, is remarked; but when small, they
remain hot for some time."--_Dr. Phipson._
[16] Silliman's Journal, September, 1864.
[17] The same explanation is given by T. M. Hall, F.G.S., in the
Popular Science Review for Oct. 1866.
[18] This list contains nothing but _aerolites_. In the Edinburgh
Review for January, 1867, we find the following statements: "Out of
the large number of authentic aerolites preserved in mineralogical
collections, two only--one on the 10th of August, and one on the 13th
of November--are recorded to have fallen on star-shower dates. On the
other hand, five or six meteorites, on the epoch of the 13th-14th of
October, belong to a date when star-showers, so far as is at present
known, do not make their appearance." The inaccuracy of the former
statement is sufficiently apparent. In regard to the latter we remark
that Quetelet's Catalogue gives one star-shower on the 14th of October,
and another on the 12th.
[19] The date of this remarkable occurrence is worthy of note as a
probable aerolite epoch. From the 12th to the 15th of March we have the
following falls of meteoric stones:
1. 1731, March 12th. At Halstead, Essex, England.
2. 1798, March 12th. At Salés, France.
3. 1806, March 15th. At Alais, France.
4. 1807, March 13th. At Timochin, Russia.
5. 1811, March 13th. At Kuleschofka, Russia.
6. 1813, March 13th-14th. The phenomena above described.
7. 1841, March 12th. At Grüneberg, Silesia.
Numerous fire-balls have appeared at the same epoch.
[20] The innermost or semi-transparent ring of Saturn appears to be
similarly constituted, as the body of the planet is seen through it
without any distortion whatever.
[21] Origin of the Stars, p. 173.
[22] Origin of the Stars, p. 184.
[23] Since the above was written Prof. Ennis has informed the author
that, without making any estimate of his own, he adopted the density of
Jupiter's first satellite as given in Lardner's _Handbook of Astronomy_.
[24] Origin of the Stars, p. 77.
[25] Youman's Correlation and Conservation of Forces, p. 244.
[26] Iowa Instructor and School Journal for November, 1866, p. 49.
[27] A recent hypothesis in regard to the temporary star of 1572 has
been proposed by Alexander Wilcocks, M.D., of Philadelphia. See Journ.
Acad. Nat. Sci. of Phila. for 1859.
[28] Gautier's Notice of Recent Researches relating to
Nebulæ.--Silliman's Journal for Jan. 1863, and March, 1864.
[29] Outlines of Astronomy, Art. 442.
[30] A learned and highly interesting examination of this hypothesis
will be found in a memoir "On the Secular Variations and Mutual
Relations of the Orbits of the Asteroids," communicated to the Am.
Acad. of Arts and Sciences, April 24th, 1860, by Simon Newcomb, Esq.
[31] For an explanation of the origin of the asteroids according to
the nebular hypothesis, see an article by David Trowbridge, A.M., in
Silliman's Journal for Nov. 1864, and Jan. 1865.
[32] H. C. Sorby, F.R.S.
[33] Harte's Trans. of Laplace's Syst. of the World, vol. ii., note vii.
[34] Grant's Hist. of Phys. Astr., p. 302.
[35] Nos. 1632 and 1633.
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Transcriber's Notes:
Punctuation and spelling were made consistent when a predominant
preference was found in this book; otherwise they were not changed.
Simple typographical errors were corrected; occasional unbalanced
quotation marks retained.
Ambiguous hyphens at the ends of lines were retained.
Text uses both "star shower" and "star-shower"; not changed here.
"Keppler" is spelled that way in this text.