A century's progress in astronomy

A Century's Progress in
Astronomy

A Century's Progress

IN

Astronomy

BY

HECTOR MACPHERSON, JUN.

MEMBER OF THE SOCIETfi ASTRONOMIQUE DE FRANCE ',

MEMBER OF THE SOCIETE BEI.GE D'ASTRONOMIE ',

AUTHOR OF 'ASTRONOMERS OF TO-DAY'

WILLIAM BLACKWOOD AND SONS

EDINBURGH AND LONDON

MCMVI

All Rights reserved

PKEFACE.

THE present volume originated in a desire to
present, in small compass, a record of the mar-
vellous progress in astronomy during the past
hundred years. Indebtedness should be acknow-
ledged to the valuable works of Professor New-
comb, Professor Schiaparelli, Professor Lowell,
Professor Young, Sir Robert Ball, Mr Gore, M.
Flammarion, and Miss Clerke, who, as the
historian of modern astronomy, occupies a place
at once authoritative and unique.

Portions of Chapters II. and XII. have already
appeared in the form of an article on the Con-
struction of the Heavens, contributed by the
writer to the American periodical, 'Popular
Astronomy. '

BALERNO, MID-LOTHIAN,
October 1906.

284153

CONTENTS.

CHAPTER I.

HERSCHEL THE PIONEER.

PAOE

Influence of Herschel's work — His characteristics — Birth and
early years — Emigration to England — Caroline Herschel
— Discovery of Uranus — King's Astronomer — Latter years
and death — Death of Caroline Herschel 1

CHAPTER II.

HERSCHEL THE DISCOVERER.

Solar researches — Study of Venus — Of Mars — The Asteroids
— Jupiter — Saturn — Discovery of satellites — Uranian satel-
lites— Cometary researches — Motion of the Solar System —
Discovery of binary stars — Clusters and nebulae — Nebulous
stars — The Nebular Hypothesis — Star-gauging — The disc-
theory — Subordinate clusters — Abandonment of the disc-
theory — Second method of star -gauging — Estimate of
Herschel's work 15

CHAPTER III.

THE SUN.

Schwabe and the sun-spot period — Researches of Wolf,
Lamont, Sabine, Gautier — Observations of Carrington

Vlll CONTENTS.

and Sporer — Career and work of Fraunhofer — Spectrum
analysis — Work of Kirchhoff — Solar eclipse work — The
Solar prominences — Janssen and Lockyer — Huggins and
Zollner — Work of Young — The Italian spectroscopists,
Secchi, Eespighi, Tacchini — Career of Tacchini — The re-
versing layer — The Corona — Doppler's principle — Rota-
tion of the Sun — Work of Dune"r — Janssen's solar atlas —
Maunder and magnetism — Solar theories — Distance of the
Sun — Summary . . . . . .43

CHAPTEE IV.

THE MOON.

Life and work of Schroter — Of Madler— Of Schmidt-
Changes on the Moon — Selenography in England —
Lunar atmosphere — Lunar photography — Work of W. H.
Pickering — The new Selenography — The Moon's heat —
Motion of the Moon — Acceleration of the Moon's mean
motion — Work of Laplace, Adams, Delaunay . . 65

CHAPTER V.

THE INNER PLANETS.

The problem of Vulcan — Mercury — Work of Schroter —
Schiaparelli, his life and work — Work of Lowell —
Spectrum of Mercury — Venus — Rotation period : work
of Schroter, Di Vico, Schiaparelli, Tacchini, Lowell —
Atmosphere and surface of Venus — The Earth : variation
of latitude — Mars — Rotation of Mars — Surface — Discovery
of canals — Work of Schiaparelli and Lowell — Interpreta-
tion of the canals — The theory of intelligent life — Spectrum
of Mars— Satellites— The Asteroids— Bode's law— Work
of Piazzi and Olbers — Application of photography by Wolf
— Discovery of Eros . . . . . .80

CONTENTS. IX

CHAPTEE VI.

THE OUTER PLANETS.

Physical condition of Jupiter — Work of Zollner and Proctor
—The red spot— Satellites— Discovery of fifth satellite —
Sixth and seventh satellites — Eings of Saturn : Bond,
Maxwell, Keeler— Struve's theory — Globe of Saturn —
New satellites — Uranus and its satellites — Discovery of
Neptune — Adams and Le Verrier — Satellite — Trans -
Neptunian planets ...... 103

CHAPTEE VII.

COMETS.

Life and work of Olbers — His repulsion theory — Life and
work of Encke — His comet — Biela's comet — Faye's comet
— Eeturn of Halley's comet— Donati's comet— Comet of
1861 — Spectroscopic study of comets — Theory of Bredik-
hine— Spectra of comets— Comets of 1880 and 1882— The
capture theory — Cometary photography . . .123

CHAPTEE VIII.

METEORS.

Meteoric shower of 1833— Work of Olmsted— Work of Erman
and Kirkwood — Of H. A. Newton — Adams and the
meteoric orbit — Shower of 1866 — Connection of comets
and meteors — Work of Schiaparelli — Shower of meteors in
1872 — 'Le Stelle Cadenti' — Meteoric observation — A. S.
Herschel — Work of Denning — Stationary radiants —
Bolides and aerolites — Origin of aerolites . . .138

CONTENTS.

CHAPTEK IX.

THE STARS.

Distance of the stars — Life and work of Bessel — Studies of
Struve — Life and work of Henderson — Work of Peters,
Otto Struve, Brunnow, and Ball— Measures of Gill—
Parallax of first-magnitude stars — Relative and absolute
parallax — Work of Kapteyn — Application of photography
— Star- catalogues — Argelander's 'Durchmusterung' — Work
of Schonfeld— Work of Gould— The ' Cape Photographic
Durchmusterung ' — Work of Gill and Kapteyn — Interna-
tional chart of the heavens — Work of Peck — Proper
motions of the stars — Star-drift — Discoveries of Proctor
and Flammarion — Radial motion — Work of Huggins,
Vogel, and Campbell — Solar motion . . .150

CHAPTER X.

THE LIGHT OF THE STARS.

Work of Fraunhofer and Donati — Life and work of Secchi
— His types of spectra — Life and work of Huggins —
Photography of spectra — Life and work of Vogel — His
classification of spectra — Work of Duner — Of Pickering —
Spectroscopic catalogues — Analysis of spectra — Stellar
photometry — Life and work of E. C. Pickering — Variable
stars — Work of Goodricke — Of Argelander, Schmidt, Heis,
Schonfeld — Studies of Duner — Of Gore — Photographic
discoveries — Classification — Algol variables : their explan-
ation — Explanation of other variables — r\ Argus —
Temporary stars— Of 1848— Of 1866— Of 1876— Of 1885
— Of 1892 — Photographic discoveries — Nova Persei, 1901
—New star of 1903— Theories of temporary stars . .169

CONTENTS. XI

CHAPTER XL

STELLAR SYSTEMS AND NEBULAE.

Life and work of John Herschel — Binary stars— Computation
of orbits— Work of Wilhelm Struve— Of Otto Struve— Of
Burnham — Satellites of Sirius and Procyon — Astronomy
of the invisible — Work of Pickering and Vogel — Spectro-
scopic binaries — Work of Be*lopolsky and Campbell — Star-
clusters — Nature of nebulae — Spectroscopic work of
Huggins — Of Copeland — Nebular photography — Work
of Eoberts, Barnard, Wolf— Of Keeler . . .197

CHAPTER XII.

STELLAR DISTRIBUTION AND THE STRUCTURE OF THE
UNIVERSE.

Work of John Herschel — Researches of Wilhelm Struve —
Extinction of light — Madler's " central sun " — Distribution
of nebulae — Work of Proctor — Aggregation of stars on the
Galaxy — Work of Gore and Schiaparelli — Studies of
Gould — Researches of Kapteyn — Of Newcomb — Is the
Universe limited 1 Newcomb's argument — Observations of
Celoria — Researches of Seeliger — External Universes —
Gore's speculations . . . . . .214

CHAPTER XIII.

CELESTIAL EVOLUTION.

Laplace's nebular hypothesis — Helmholtz and solar con-
traction— Theories of solar heat — Objections to La-
place's theory — Faye's hypothesis — Ball's exposition — The
meteoritic theory of Proctor — Its extension by Lockyer —
Evolution of the stars — Vogel's order of evolution — Tidal
friction : work of Darwin — See's explanation of double
stars — Future of the Universe 227

A CENTURY'S PROGRESS IN
ASTRONOMY,

CHAPTER I.

HERSCHEL THE PIONEER.

IN astronomy, as in other sciences, the past
hundred years has been a period of unparalleled
progress. New methods have been devised,
fresh discoveries have been made, new theories
have been propounded ; the field of work has
widened enormously. In fact, the science of
the heavens has become not only boundless in
its possibilities, but more awe-inspiring and
marvellous.

To whom in the main is this great advance
due ? To the great pioneer of what may be
called modern astronomy — William Herschel.
Not only did Herschel reconstruct the science
and widen its bounds, but his powerful genius

2 \ 'A. .'CBKOTRY'S PROGRESS IN ASTRONOMY.

directed the course of nineteenth century re-
search. As an astronomical observer he has
never been surpassed. In the breadth of his
views he was equalled only by Newton ; and
indeed he excelled Newton in his unwearied
observations and his sweeping conceptions of
the Universe. To quote his own remark to the
poet Campbell, he " looked farther into space
than ever human being did before him."

Herschel studied astronomy in all its aspects.
In all the branches of modern astronomy he was a
pioneer. He observed the Sun, Moon, and planets,
devoting special attention to Mars and Saturn.
He doubled the diameter of the Solar System
by the discovery of Uranus. He discovered
several satellites and studied comets. He was
pre-eminently the founder of sidereal astronomy.
He discovered binary stars, thus tracing the law
of gravitation in the distant star -depths; while
to him is due the credit of the discovery of the
motion of the Solar System. He founded the
study of star-clusters and nebulae, propounded
the nebular hypothesis, and devised two methods
of star -gauging. Above all, he was the first
to attempt the solution of one of the noblest
problems ever attacked by man — the structure
of the Universe. In fact, the latter problem
was the end and aim of his observations. As

HEKSCHEL THE PIONEER. 3

Miss Clerke remarks, "The magnificence of the
idea, which was rooted in his mind from the
start, places him apart from and above all pre-
ceding observers." Most of the departments
of modern astronomy find a meeting -place in
Herschel, as the branches run to the root of
the tree. He discussed astronomy from every
point of view. Before, however, proceeding to
examine the work of this great man, it is well
to note a few of his characteristics. These
characteristics, once understood, give us the
key to his researches. Before we can master
Herschel the astronomer we must understand
Herschel the man.

Notwithstanding the fact that Herschel spent
most of his life in England, and that he is in-
cluded in the ' Dictionary of National Biography/
he was pre-eminently a German. Like most
Germans his style of writing was somewhat
obscure, and this was emphasised when he wrote
in English, owing to his imperfect command of
the language. Had he written in German as
well as in English, he would probably have been
better understood in his native country, where
erroneous views of his theories were long enter-
tained. Even so distinguished an astronomer
as Wilhelm Struve, when translating Herschers
papers into German, made a mistake when

4 A CENTURY'S PROGRESS IN ASTRONOMY.

translating a certain passage, which leaves the
erroneous impression that Herschel believed the
Universe to be infinite — a mistake which would
not have arisen had he written in German.

The student of Herschel should also be careful
in quoting the views of the great astronomer.
Had Herschel at the close of his life written
a volume containing his final views on the con-
struction of the heavens, this would not have
been necessary ; but Herschel did not write such
a volume. His researches were embodied in a
series of papers communicated to the Royal
Society from 1780 to 1818. As. he observed
the heavens his opinions progressed, so that
a statement of his views at any given time was
by no means a statement of his final opinions.
The late R. A. Proctor, who was the first great
exponent of Herschel in England, has well said :
" It seems to have been supposed that his papers
could be treated as we might treat such a work
as Sir J. Herschel's ' Outlines of Astronomy ' ;
that extracts might be made from any part of
any paper without reference to the position
which the paper chanced to occupy in the
entire series."

Herschel, like the true student of nature, held
theories very lightly. They were to him but
roads to the truth. Unlike many scientists,

HERSCHEL THE PIONEER. 5

he did not interpret observations by hypothesis :
he framed his theories to fit his observations. If
he found that a certain theory did not agree
with what he actually saw in the heavens, he
abandoned it : he did not hesitate to change
his views as his investigations proceeded. " No
fear of ' committing himself,' " says Miss Clerke
in her admirable work on * The Herschels/ " de-
terred him from imparting the thoughts that
accompanied his multitudinous observations. He
felt committed to nothing but truth/'

In the mind of Herschel imagination and ob-
servation were marvellously blended. He was a
philosophical astronomer. Although his imagina-
tion was a very vivid one he did not allow his
fancies to run away with him, as Kepler some-
times did : on the other hand, he did not, like
Flamsteed, refrain from speculating altogether.
"We ought," he wrote in 1785, "to avoid two
opposite extremes. If we indulge a fanciful im-
agination, and build worlds of our own, we must
not wonder at our going wide from the path
of truth and nature. On the other hand, if
we add observation to observation, without at-
tempting to draw not only certain conclusions
but also conjectural views from them, we offend
against the very end for which only observations
ought to be made."

6 A CENTURY'S PROGRESS IN ASTRONOMY.

These characteristics — the lightness with which
he held his theories, his vivid imagination, and
his philosophical reasoning — are the secrets of
Herschel's success as an astronomer. Nearly
all his ideas and speculations have been con-
firmed. As Arago has said, " We cannot but
feel a deep reverence for that powerful genius
that has scarcely ever erred." Herschel, like
all other great students of Nature, was deeply
religious. He could not observe the heavens
without feeling awed at the marvels which his
telescopes revealed. In his own words, "It is
surely a very laudable thing to receive instruc-
tion from the Great Workmaster of Nature."

Friedrich Wilhelm Herschel, born in Hanover
on November 15, 1738, was the fourth child of
Isaac Herschel, an oboist in the band of the
Hanoverian Guard. Isaac Herschel, a native
of Dresden, was an accomplished musician, and
all his children, ten in number, inherited his
talent. Of these ten, six survived, and only
two became famous. These were William, the
great astronomer, and his sister Caroline (born
on March 16, 1750), who became a student of
the heavens only second to her brother.

At the garrison school in Hanover, where
the Herschels were educated, William Herschel
showed intense love and aptitude for learning,

HERSCHEL THE PIONEER. 7

and was more diligent and persevering than his
brother Jacob, his senior by four years. In
1753 he became oboist in the band of the
Hanoverian Guard in which his father was
now bandmaster. In her valuable memoirs, his
sister relates that her father was very interested
in astronomy, and that he taught his children
the names of the constellations. William became
devoted to the science, and constructed a small
celestial globe on which equator and ecliptic
were engraved. But his studies were much
hampered. His mother had a great dislike to
learning : she had no sympathy with aspira-
tions, and tried to prevent her children becom-
ing well educated. Above all, the Hanoverian
Guard was ordered to England in 1755, when
a French invasion was feared, and to that
country Herschel proceeded, along with his
father and brother.

Returning to Germany in 1756, the Hano-
verian Guard was employed the following year
in the Seven Years' War. Hanover was invaded
by the French, and, conscription being the rule,
the musicians were not exempted from service.
Under the command of the Duke of Cumber-
land the Guard suffered a terrible defeat at
Hastenbeck. William Herschel spent the night
after the battle in a ditch, and decided that

8 A CENTURY'S PROGRESS IN ASTRONOMY.

soldiering would not be his profession. He
deserted, and, with the consent of his parents,
he sailed for England. After his arrival at
Dover, he wandered through the country in
search of musical employment. At length, in
1760, he was appointed to train the band of
the Durham Militia, and four years later paid a
secret visit to Hanover, where he was welcomed
by his father, whose health was now failing,
and by his sister Caroline. In the following
year he was promoted to the post of organist
at Halifax, and in 1766 he removed to Bath as
oboist in Linley's Orchestra. Finally, in 1767,
he became organist in the new Octagon Chapel
at Bath. Herschel was now twenty-nine years
old, and known as a famous musician. As Miss
Clerke remarks : " The Octagon Chapel soon
became a centre of fashionable attraction, and
he soon found himself lifted on the wave of
public favour. Pupils of high rank thronged to
him, and his lessons often mounted to thirty-
five a- week."

In the year of his appointment his father died,
aged sixty, after a life of trouble and hardship.
His death was a great blow to his daughter
Caroline, whom he had educated when her
mother was from home. Caroline Herschel was
naturally possessed of musical ability, but her

HERSCHEL THE PIONEER. 9

mother and elder brother had determined that
she should be a housemaid, — a determination
which William, who was devotedly attached to
his sister, opposed. Finally, in 1772, he visited
Hanover, and took his sister to England with
him to act as his housekeeper. But for her
unwearied devotion it is doubtful whether
William Herschel would have become the great
astronomer.

About the time of his appointment in Bath
Herschel commenced the study of languages and
mathematics, reading Maclaurin's ' Fluxions ' and
Ferguson's ' Astronomy.' The perusal of the
latter volume revived his love for astronomy.
After fourteen or sixteen hours' teaching he
would retire to his bedroom and read of the
wonders of the heavens. His interest increased
as he proceeded, until, in his own words, "I
resolved to take nothing upon trust, but to
see with my own eyes all that other men had
seen before me." Accordingly he hired a small
reflector. Inquiring the price of a larger instru-
ment, he found it to be quite beyond his means.
Then in 1772, when his sister came to keep his
house for him, he resolved to make his own
telescope. First he tried the fitting of lenses
into pasteboard tubes, but this being a total
failure, he bought the apparatus of a Quaker

10 A CENTURY'S PROGRESS IN ASTRONOMY.

optician who had constructed, or attempted to
construct, reflecting telescopes. In June 1773,
assisted by his sister and by his brother Alex-
ander, then in Bath, he commenced work. His
first speculum mirror was five inches in diameter ;
and, while it was in process of construction, he
was obliged to hold his hands on it for sixteen
hours at a stretch, while his sister supplied
his food and read 'The Arabian Nights/ * Don
Quixote/ and other tales aloud to him to pass
the time. At last, after two hundred failures,
he finished a 5-inch reflector, and on March 4,
1774, he observed the Orion nebula. No sooner
had Herschel commenced his celestial explora-
tions than he resolved to survey the entire
heavens, leaving no spot unvisited.

In 1775 he commenced his review of the
heavens, but finding his telescope inadequate
he began the work of telescope -making afresh.
Meanwhile he had much to distract him from
astronomy. In 1776 he became director of the
Public Concerts at Bath. Yet his enthusiasm
was unbounded : he would run to his house
between the acts at the theatre to observe the
heavens. In 1779, when observing the Moon
from the street in front of his house, a gentle-
man asked permission to see the celestial
wonders, a request which Herschel granted.

HERSCHEL THE PIONEER. 11

The gentleman, Dr Watson of Bath, introduced
Herschel to the Literary Society, and we find
him in 1780 contributing two papers to the
Royal Society on Mira Ceti and the Moon. In
the same year he commenced his second review
of the heavens, and during its progress he made
his first great discovery. On March 13, 1781,
while surveying the constellation Gemini, he
discovered a faint object distinguished by a
disc, which he concluded to be a tailless comet,
but which was soon shown to be a new planet
beyond the orbit of Saturn. This was the first
planetary discovery made within the memory of
man. King George III. summoned Herschel to
London, and gave him a pension of £200 a-year,
with the title of King's Astronomer, pardoning
him also for his desertion from the army more
than twenty years previously. Herschel then
named the new planet the " Georgium Sidus,"
a title now abandoned and replaced by Uranus.
William and Caroline Herschel now moved to
Datchet, near Windsor, in 1785 to Clay Hall,
and finally, in 1786, to Slough, — "the spot of
all the world," said Arago, " where the greatest
number of discoveries have been made." Here
Herschel and his sister worked for nearly forty
years. He communicated to the Royal Society
paper after paper on astronomy in all its aspects.

12 A CENTURY'S PROGRESS IN ASTRONOMY.

He also continued the work of telescope-making,
and constructed, in 1789, his 40-foot reflector,
the wonder of the age. In 1787 his sister
was appointed his assistant, and together the
Herschels worked from dusk to dawn. Caroline
Herschel herself detected eight comets and
numerous nebulae. She relates in her memoirs
that on one occasion, while she was acting as
assistant, the ink froze in her pen. But such
inconveniences mattered not to the Herschels.
As Miss Clerke has well remarked, " Every
serene dark night was to him a precious oppor-
tunity, availed of to the last minute. The
thermometer might descend below zero, ink
might freeze, mirrors might crack ; but, pro-
vided the stars shone, he and his sister worked
on from dusk to dawn. . . . On one occasion he
is said to have worked without intermission
at the telescope and the desk for seventy-two
hours."

Honours were showered on Herschel. He was
knighted in 1816, and became President of the
Royal Astronomical Society in 1820, besides
receiving several honorary degrees. But honours
in no way elated him. Advancing years in no
way affected his wonderful mind. But his duties
as King's Astronomer necessitated his acting as
" showman of the heavens " on the visits of

HERSCHEL THE PIONEER. 13

royalties to Windsor, often after a whole day's
work, when rest was absolutely necessary. This
tremendous strain, which reflects little credit on
the Court, proved too much for the old man.
His health began to give way, although his
mind was as vigorous as ever.

Herschel contributed his last paper to the
Royal Society in 1818, and three years later
sent a list of double stars to the new Astro-
nomical Society. He made his last observation
on June 1, 1821. His strength had now left
him, and to this he could not reconcile himself.
As Miss Clerke puts it, "All his old instincts
were still alive, only the bodily power to carry
out their behests was gone. An unparalleled
career of achievement left him unsatisfied with
what he had done. . . . His strong nerves
were at last shattered." After a prolonged
period of failing health he died at Slough, at
the age of eighty-three, on August 25, 1822.
On September 7 he was buried in the church-
yard of St Laurence at Upton. On his tomb-
stone are engraved the words — " Ccelorum perru-
pit claustra" — he broke through the barriers of
the skies.

The death of her brother was a terrible blow
to Caroline Herschel. Expecting to live only
a twelvemonth, she returned to Hanover to

14 A CENTURY'S PROGRESS IN ASTRONOMY.

the home of her brother, Dietrich Herschel.
But she lived twenty-five years among people
who cared nothing for astronomy. She was
delighted at Sir John Herschel's continuation
of his father's work. She compiled a catalogue
of all the clusters and nebulao observed by her
brother, for which she received the gold medal
of the Astronomical Society, and she was created
an honorary member. In 1846 she received from
the King of Prussia the gold medal of science.
But no honours made her in any way elated.
She always held that whoever said much of
her said too little of her brother. After a pro-
longed decline of health, she died on January 9,
1848, aged ninety -seven years, and was buried
beside her father in the churchyard of the
Gartengemeinde at Hanover, leaving behind her
a noble example of self-sacrifice and devotion.

CHAPTER II.

HERSCHEL THE DISCOVERER.

ONE result of Herschel's discoveries among the
stars and nebulae is that his studies of the Sun
and planets, with the exception of the discovery
of Uranus, have been completely thrown into
the shade. Nevertheless, his work in solar
and planetary astronomy alone would have
gained for him a higher position in astronomy
than his contemporaries. The planets, satellites,
and comets were all attentively studied by the
great astronomer ; indeed, the scientific inves-
tigation of the surfaces of Mars and Saturn
began with Herschel.

" His attention to the Sun," Miss Clerke truly
remarks, " might have been exclusive, so diligent
was his scrutiny of its shining surface." Sun-
spots were specially investigated by Herschel,
who closely studied their peculiarities, regarding
them as depressions in the solar atmosphere.
He also paid much attention to the faculse, but

16 A CENTURY'S PROGRESS IN ASTRONOMY.

could not observe them to the north and south
of the Sun, thus proving their connection with
the spots which are confined to the regions
north and south of the equator. " There is all
over the Sun a great unevenness," said Herschel,
"which has the appearance of a mixture of
small points of an unequal light ; but they are
evidently a roughness of high and low parts."

Herschel's solar observations were very valu-
able, and did much for our knowledge of the
orb of day. His theory of the Sun's constitu-
tion— a development of the hypothesis put
forward by Alexander Wilson (1714-1786), Pro-
fessor of Astronomy in Glasgow — was, however,
very far from the truth. This was almost the
only instance in which Herschel was mistaken.
He regarded the Sun as a cool, dark globe,
" a very eminent, large, and lucid planet,
evidently the first, or, in strictness of speaking,
the only primary one of our system." In his
opinion an extensive atmosphere surrounded the
Sun, the upper stratum forming what Schroter
named the "photosphere." This atmosphere,
estimated as two or three thousand miles in
depth, was regarded as giving out light and
heat. Below this shining atmosphere there
existed, Herschel believed, a region of clouds
protecting the globe of the Sun from the

HERSCHEL THE DISCOVERER. 17

glowing atmosphere, and reflecting much of
the light intercepted by them. The spots were
believed to be openings in these atmospheres,
caused by the action of winds, the umbra or
dark portion of the spot thus representing the
globe of the Sun, which Herschel believed to
be " richly stored with inhabitants." This theory
held its ground for many years. Newton, it is
true, believed the Sun to be gaseous, but he
propounded no hypothesis of its constitution.
Herschel's theory, on the other hand, was
fully developed, plausible, and attractive. It
was held by eminent men of science until
1860, when the revelations of the spectroscope
showed it to be quite untenable. The theory
was supported for many years by Sir John
Herschel, who, however, abandoned it in 1864.
Herschel made several attempts to ascertain
whether any connection existed between the
state of the Sun and the condition of the
Earth. In 1801 he was inclined to believe
that " some temporary defect of vegetation "
resulted from the absence of sun-spots, which,
he thought, "may lead us to expect a copious
emission of heat, and, therefore, mild seasons."
Herschel believed, in fact, that food became
dear at the times of spot-minima. It may be
remarked that Herschel never noted the spot-
is

18 A CENTURY'S PROGRESS IN ASTRONOMY.

period of eleven years, the discovery of which
was afterwards made by Schwabe.

Herschel closely scrutinised the surfaces of
the planets. Mercury alone was neglected by
him. From 1777 to 1793 he observed Venus,
with the object of determining the rotation
period, but he was unable to observe any
markings on the surface of the planet. He
did not place reliance on Schroter's value of
the rotation period (about twenty-three hours).
Meanwhile, Schroter announced the existence
on Venus of mountains which rose to five or
six times the height of Chimborazo. As to
these, said Herschel, " I may venture to say
that no eye which is not considerably better
than mine, or assisted by much better instru-
ments, will ever get a sight of them." Herschel
demonstrated the existence of an extensive
atmosphere round Venus.

" The analogy between Mars and the Earth,"
Herschel wrote in 1783, "is perhaps by far the
greatest in the whole Solar System." In 1777
he began, in his house at Bath, a series of
observations on the red planet, which yielded
results of the utmost importance. Fixing his
attention on the white spots at the north and
south poles, — discovered by Maraldi, nephew of
Cassini, — he soon ascertained the fact that they

HERSCHEL THE DISCOVERER. 19

waxed and waned in size, the north polar cap
shrinking during the summer of the northern
hemisphere, increasing in winter, and vice versa
in the southern hemisphere. He regarded the
caps as masses of snow and ice deposited from
"a considerable, though moderate, atmosphere,"
a theory now generally accepted. Herschel gave
an immense impetus to the study of Mars. He
carefully examined the planet's surface, and the
dark markings were regarded by him as oceans.

During Herschel's lifetime the four small
planets, Ceres, Pallas, Juno, and Vesta, were
discovered by Piazzi, Olbers, and Harding. The
great astronomer was much interested in these
small worlds. He commenced a search through
the Zodiacal constellations for new planets, but
failed. He was of opinion that many minor
planets would be discovered. Accepting Gibers'
theory of the disruption of a primitive planet,
Herschel calculated that Mercury might be
broken up into 35,000 globes equal to Pallas.
Meanwhile Herschel named the four new planets
"Asteroids," owing to their minute size. He
estimated the diameter of Ceres at 162 miles
and Pallas at 147 miles, but Professor Barnard's
measures have shown them to be larger.

In connection with the discovery of the
Asteroids, Herschel showed a very fine spirit.

20 A CENTURY'S PROGRESS IN ASTRONOMY.

In c The Edinburgh Review ' Brougham declared
that Herschel had devised the word " asteroid,"
so that the discoveries of Piazzi and Olbers
might be kept on a lower level than his own
discovery of Uranus. Many scientists would
have been much offended at this contemptible
insult, but Herschel merely remarked that he
had incurred "the illiberal criticism of 'The
Edinburgh Review/ " and that the discovery of
the Asteroids " added more to the ornament of
our system than the discovery of another planet
could have done."

In Herschel's time astronomers were acquainted
with three of the outer planets, — Jupiter, Saturn,
and Uranus, — all of which were closely studied
by the great astronomer. The belts of Jupiter
were supposed by him to be analogous to the
" trade-winds " in the atmosphere of the Earth ;
while the drifting -spots on Jupiter's disc and
their irregular movements were carefully noted.
His observations on the four satellites of Jupiter
led him to believe that, like our Moon, they
rotated on their axes in a period equal to that
of their revolution round their primary — an
opinion shared by Laplace, and by many modern
astronomers.

Herschel's researches regarding Saturn were,
however, much more important than those on

HERSCHEL THE DISCO VEBER. 21

Jupiter. The globe of the planet, the rings
and the satellites, were favourite objects of
study at Bath and Slough. In 1794 he per-
ceived a spot on the surface of Saturn, and
made the first determination of the rotation of
the planet, which he fixed as 10 hours 16
minutes, — a result confirmed by modern astron-
omers. The rings were subjected to the closest
scrutiny. Herschel believed them to be solid,
and he also considered them to revolve round
Saturn in about 10 hours. It appears that he
observed the famous " dusky ring," but supposed
it to be a belt on the surface of the planet.
He also studied Cassini's division in the ring,
ascertaining its reality.

On completing his famous 40 -foot reflector,
Herschel, on August 28, 1789, turned it on
Saturn and its five known satellites. Near the
planet, and in the plane of the ring, was seen
another object, which Herschel believed to be
a sixth satellite. To settle the question, he
watched the planet for several hours to see if
the object would partake in the planet's motion.
Finding that it did, he announced it as a new
satellite, which he found to revolve round
Saturn in 1 day 8 hours. About three weeks
later, on September 17, Herschel discovered
another satellite yet closer to Saturn, revolving

22 A CENTURY'S PROGRESS IN ASTRONOMY.

round the planet in about 22 hours. These
two satellites were not seen by any astronomers
except Herschel ; and after his death they could
not be observed. His son, however, rediscovered
them.

The eighth satellite, Japetus, was shown by
Herschel to rotate on its axis in a period equal
to that of its revolution, and his observations
were confirmed by modern observers. " I can-
not," Herschel said, "help reflecting with some
pleasure on the discovery of an analogy which
shows that a certain uniform plan is carried
on among the secondaries of our Solar System ;
and we may conjecture that probably most of
the satellites are governed by the same law."
In April 1805 Herschel observed the globe of
Saturn to present not a spherical but a "square-
shouldered" aspect. It was for long believed
that this was an optical illusion ; but Proctor
and others have shown that it is quite possible
for storms in Saturn's atmosphere to cause the
planet's apparent distortion in shape.

Herschel paid much attention to the planet
Uranus, which he discovered on March 13, 1781.
The discovery of Uranus, which was mentioned
in a previous chapter, was in a sense the most
striking of Herschel's achievements. Uranus was
the first planet discovered within the memory

HERSCHEL THE DISCOVERER. 23

of man : besides, the discovery enlarged the
diameter of the Solar System from 886 to
1772 millions of miles. Throughout his life-
time Herschel referred to the planet as the
"Georgium Sidus," out of gratitude to George
III. for appointing him King's Astronomer; but
the astronomers of France and Germany, who,
as Sir Robert Ball remarks, " saw no reason
why the King of England should be associated
with Jupiter and Saturn," opposed this term.
Lalande called the planet " Herschel," but
Herschel's countrymen, the Germans, named it
Uranus, in keeping with the custom of desig-
nating the planets from the Greek mythology.
The name of Uranus ultimately prevailed.

In January 1787 Herschel discovered two
satellites to Uranus, with the aid of his 20-
foot telescope. These satellites he believed to
revolve round Uranus in 8 days and 13 days
respectively, and accordingly he made a draw-
ing of what their positions should be on
February 10. On that day he found them in
their predicted places. In 1797 he announced
that the satellites revolved round Uranus in
orbits at right angles to the ecliptic, and in
a retrograde direction. In subsequent years
Herschel believed that he had discovered other
four satellites to Uranus, but he was unable

24 A CENTUKY'S PROGRESS IN ASTRONOMY.

to confirm his belief. As Mr Gore says, some
of the satellites " must, therefore, have been
either optical 'ghosts' or else small fixed stars
which happened to be near the planet's path at
the time of observation. Herschel also suspected
that he could see traces of rings round Uranus
like those round Saturn, but his observation
was never confirmed, either by himself or other
observers."

Although Herschel made several important
observations on the Moon, and measured the
heights of the lunar mountains, he was not
a devoted student of our satellite. Caroline
Herschel remarks in her memoirs that if it
had not been for clouds or moonlight, neither
her brother nor herself would have got any
sleep ; adding that Herschel on the moonlight
nights prepared his papers or made visits to
London. However, he did make some investiga-
tions, and in 1783 and 1787 believed himself
to have witnessed the eruption of three lunar
volcanoes. He afterwards concluded, however,
that what he believed to be eruptions was really
the reflexion of earth - shine from the white
peaks of the lunar mountains. Herschel never
discovered a comet, leaving that branch of
astronomy to his sister, who discovered eight
of these objects. He was, however, much in-

HERSCHEL THE DISCOVERER. 25

terested in comets, and attentively studied them,
introducing the terms of " head," " nucleus," and
"coma." Herschel anticipated the view that
comets are not lasting, but are partly disin-
tegrated at their perihelion passages. He was
of opinion that they travelled from star to star.
The extent of their tails and appendages he
thought to be a test of their age.

We have now completed our sketch of
Herschel's important labours regarding our
Solar System. As Miss Clerke says, "A whole
cycle of discoveries and successful investigations
began and ended with him." But through ob-
serving the stars he made a further discovery
in connection with the Solar System; indeed,
one of the greatest discoveries in the history
of astronomy — the movement through space of
the Sun, carrying with it planets and comets.

"If the proper motion of the stars be ad-
mitted," said Herschel, "who can deny that of
our Sun?" Of course it was plain that the
motion of the Sun could only be detected
through the resulting apparent motion of the
stars. Thus, if the Sun is moving in a certain
direction, the stars in front will appear to open
out, while those behind will close up. But the
problem is by no means so easy as this. The
stars are also in motion, and, before the solar

26 A CENTURY'S PROGRESS IN ASTRONOMY.

motion can be discovered, the proper motions of
the stars — themselves very minute — have to be
decomposed into two parts, the real motion of
the star, and the apparent motion, resulting
from the movement of the Solar System. To
any astronomer but Herschel the problem would
have been insoluble. Only sixty years had
elapsed since Halley had announced the proper
motions of the brighter stars which had been
previously supposed to be immovable — hence
the name of "fixed stars." Herschel did not
deal with the motions of many stars. Only a
few proper motions were known with accuracy
when he attacked the problem in 1783. Making
use of the proper motions of seven stars, and
separating the real from the apparent motion,
he found that the Solar System was moving
towards a point in the constellation Hercules,
the "apex" being marked by the star X Her-
culis. The rate of the solar motion, Herschel
thought, was "certainly not less than that
which the Earth has in her annual orbit." This
extraordinary discovery was one of Herschel's
greatest works. " Its directness and apparent
artlessness," Miss Clerke remarks, " strike us
dumb with wonder." In 1805 Herschel again
attacked the subject, utilising the proper motions
of thirty-six stars. His second inquiry, on the

HERSCHEL THE DISCOVERER. 2*7

whole, confirmed his previous result, the " apex "
being again situated in Hercules ; but the
determination of 1783 was probably the more
accurate of the two.

Herschel was far in advance of his time re-
garding the solar motion. The two greatest
astronomers of the next generation, Bessel and
Sir John Herschel, rejected the results reached
by Sir William Herschel. But in 1837 Argel-
ander, after a profound mathematical discussion,
confirmed Herschel's views, and proved the solar
motion to be a reality. Since that date the
problem has been attacked by various methods
by Otto Struve, Gauss, Madler, Airy, Dunkin,
Ludwig Struve, Newcomb, Kapteyn, Campbell,
and others, with the result that the reality of
the solar motion and of the direction fixed by
Herschel has been proved beyond a doubt. As
Sir Robert Ball well remarks, mathematicians
have exhausted every refinement, " but only
to confirm the truth of that splendid theory
which seems to have been one of the flashes
of Herschel's genius."

In his volume ' Herschel and his Work/ Mr
James Sime writes: "To Herschel belongs the
credit not merely of having suspected the revolu-
tion of sun around sun in the far-distant realms
of space, but also of actually detecting that this

28 A CENTURY'S PROGRESS IN ASTRONOMY.

was going on among the stars." Throughout his
career double stars were favourite objects of
observation. The study of double stars was com-
menced by Herschel while a musician in Bath.
Before his day, of course, double stars had been
discovered and studied, but it was believed that
the proximity of two stars was merely an optical
accident, the brighter star being much nearer to
us than the other. Herschel, at first sharing the
general view, observed double stars in the hope
of measuring their relative parallaxes ; assuming
one star to be much farther away from the Solar
System than another, he attempted to measure
the parallactic displacement of the brighter star
relatively to the position of the fainter. " This,"
he afterwards wrote, "introduced a new series
of observations. I resolved to examine every
star in the heavens with the utmost attention,
that I might fix my observations upon those that
would best answer my end. I took some pains
to find out what double stars had been recorded
by astronomers ; but my situation permitted me
not to consult extensive libraries, nor, indeed,
was it very material ; for as I intended to view
the heavens myself, Nature, that great volume,
appeared to me to contain the best catalogue."

Herschel, on January 10, 1782, submitted to
the Eoyal Society a catalogue of 269 double

HERSCHEL THE DISCOVERER. 29

stars : of these he himself discovered 227. In
December 1784 he forwarded another catalogue,
containing 434 stars. He soon found that he
was unable to measure stellar parallax, and the
idea dawned on him that the double stars were
physically connected by the law of gravitation,
though he made no announcement to that effect
for many years. On July 1, 1802, Herschel in-
formed the Royal Society that " casual situations
will not account for the multiplied phenomena
of double stars. ... I shall soon communicate
a series of observations, proving that many of
them have already changed their situation in a
progressive course, denoting a periodical revolu-
tion round each other." In 1803 he showed that
many stars were revolving round their centres
of gravity, proving them, in his own words, to
be "intimately held together by the bond of
mutual attraction." In other words, Herschel
discovered that the law of gravitation prevailed
in the Stellar Universe, as well as in our Solar
System — that the law which Newton ascertained
to prevail in the Solar System extended through-
out the depth of space.

Herschel did not merely prove the revolution
of the binary stars; he assigned periods to
those which he had particularly studied. He
believed the period of Castor to be 342 years;

30 A CENTURY'S PROGRESS IN ASTRONOMY.

y Leonis 1200 years ; 8 Serpentis 375 years ;
and e Bootis 1681 years. Herschel did not
compute the orbits mathematically. This was
not done for nearly thirty years, when the cal-
culation of binary star -objects was commenced
by Savary, Sir John Herschel, and Encke.

In 1782 the French astronomer, Charles
Messier (1730-1817), published a list of 103
nebulae. In the following year Herschel com-
menced his famous sweeps of the heavens with
his large reflectors, and during these he made
many remarkable discoveries. In 1786 he pub-
lished in the ' Philosophical Transactions ' of the
Royal Society a catalogue of a thousand new
nebulae and star-clusters, in which he gave the
position of each object with a short description
of its appearance, written by Caroline Herschel
while her brother actually had the object before
his eyes. In 1786 Herschel published a cata-
logue of another thousand clusters and nebulae,
followed in 1802 by a list of 500 ; making a
total of 2500 clusters and nebulae discovered by
the great astronomer. This alone would have
gained a great name for William Herschel in
this branch of astronomy. In the space of only
twenty years 2500 nebulae and clusters had been
discovered. The various nebulae and clusters
were divided into eight classes, as follows : the

HERSCHEL THE DISCOVERER. 31

first class being " bright nebulae," the second
"faint nebulae/' the third "very faint nebulae,"
the fourth " planetary nebulae," so named by
Herschel from their resemblance to planetary
discs, the fifth class contained " very large
nebulae," the sixth "very compressed and rich
clusters of stars," the seventh "pretty much
compressed clusters of large or small stars,"
and the eighth " coarsely scattered clusters of
stars."

At first Herschel believed all nebulae to be
clusters of stars, the irresolvable nebulae being
supposed to be farther from our system than
the resolvable nebulae. As many of the nebulae
which Messier could not resolve had yielded to
Herschel's instruments, Herschel believed that
increase of telescopic power would resolve the
hazy spots of light which remained nebulous. In
the paper of 1785, in which Herschel dealt with
the construction of the heavens, he stated his
belief that many of the nebulae were external
galaxies — universes beyond the Milky Way ; and
in 1786 he remarked that he had discovered
fifteen hundred universes !

Arago, Mitchel, Nichol, Chambers, and other
writers quite misinterpreted Herschel's views on
the nebulae when they said that he believed
them to be all external galaxies. In 1785

32 A CENTURY'S PROGRESS IN ASTRONOMY.

Herschel believed many to be connected with
the sidereal system ; considering that in some
parts of the Galaxy " the stars are now drawing
towards various secondary centres, and will in
time separate into different clusters." He was
coming to the view that the star-clusters were
secondary aggregations within the Galaxy, prob-
ably the true theory. He pointed out that in
Scorpio, the cluster Messier 80 is bounded by
a black chasm, four degrees wide, from which
he believed the stars had been drawn in the
course of time to form the cluster. His sister
records that one night, after a "long, awful
silence," he exclaimed on coming on this chasm
— "Hier ist wahrhaftig ein Loch im Himmel!"
(Here, truly, is a hole in the heavens.)

Herschel was now gradually giving up his
theory of external galaxies and his " disc-theory "
of the Universe ; but he still believed even the
nebulous objects to be irresolvable only through
immensity of distance. In 1791, however, he
drew attention to a remarkable star in Taurus,
surrounded by a nebulous atmosphere, regarding
which he wrote, "View, for instance, the nine-
teenth cluster of my sixth class, and afterwards
cast your eye on this cloudy star. Our judg-
ment, I will venture to say, will be that the
nebulosity about the star is not of a starry

HERSCHEL THE DISCOVERER. 33

nature. We therefore either have a central body
which is not a star, or have a star which is
involved in a shining fluid, of a nature totally
unknown to us." And with caution he added
that " the envelope of a cloudy star is more fit
to produce a star by its condensation than to
depend upon the star for its existence."

This was written in 1791, five years before
Laplace propounded his nebular theory. Mean-
while Herschel, believing that " these nebulous
stars may serve as a clue to unravel other
mysterious phenomena, found that the theory
of a " shining fluid " would suit the appear-
ance of the irresolvable planetary nebulae and
the great nebula in Orion much better than
the extravagant idea of "external universes."
Herschel now considered the Orion nebula to
be much nearer to the Solar System than he
formerly did, and ceased to regard it as ex-
ternal to the Galaxy. For twenty years Herschel
patiently observed the nebulae, and it was not
until 1811 that he propounded his nebular hypo-
thesis of the evolution of the Sun and stars.
He found the gaseous matter in all stages of
condensation, from the diffused cloudy nebulae
like that in Orion, through the planetary nebula
and the regular nebula, to the perfect stars, like
Sirius and the Sun. Herschel's nebular theory

c

34 A CENTURY'S PROGRESS IN ASTRONOMY.

was a grand conception, and a magnificent attack
on the secrets of nature.

Sir E/obert Ball says : " Not from abstract
speculation like Kant, nor from mathematical
suggestion like Laplace, but from accurate and
laborious study of the heavens, was the great
William Herschel led to the conception of the
nebular theory of evolution." Herschel's nebular
theory was wider and less rigorous than that
of Laplace. Laplace reached his theory by
reasoning backwards ; Herschel by observing the
nebulae in process of condensation. Consequently,
while Laplace's theory has required modification,
Herschel's, from its width, is universally ac-
cepted, because there is nothing mathematically
rigorous in it. The great German did not go
into details like his French contemporary. He
sketched the evolution of the stars in a wider
sense.

The astronomer's " 1500 universes," Miss Clerke
remarks, " had now logically ceased to exist."
Herschel had gathered much evidence about
nebular distribution which shattered his belief
in external universes, although he still thought
in 1818 that some galaxies were included among
the non- gaseous nebulae. In 1784 Herschel
pointed out that the clusters and nebulae " are
arranged to run in strata " ; and some time later

HERSCHEL THE DISCOVERER. 35

he found that the nebulae were aggregated near
the galactic poles ; in other words, where nebulae
are numerous, stars are scarce, and vice versa.
So rigorously did this rule hold, that when
dictating his observations to his sister Caroline,
he would, on noting a paucity of stars, warn her
to " prepare for nebulae."

" A knowledge of the construction of the
heavens has always been the ultimate object of
my observations." So Herschel wrote in 1811.
All his investigations were secondary to the
problem which was constantly before his mind
— the extent and structure of the Universe.
He aspired to be the Copernicus of the Sidereal
System. Although Bruno, Kepler, Wright, Kant,
and Lambert had speculated regarding the con-
struction of the heavens, they had not the
slightest evidence on which to base their ideas.
There was no science of sidereal astronomy. The
stars were observed only to assist navigation,
and the primary object of star-catalogues was to
further knowledge of the motions of the planets.
In Herschel's day, also, the distances of the stars
had not been measured, and he had to base his
views on the distribution of the stars. In 1784,
therefore, he commenced a survey of the heavens,
in order to ascertain the number of stars in
various parts of the sky. This method, which

36 A CENTURY'S PROGRESS IN ASTRONOMY.

he named " star -gauging," consisted in count-
ing the number of stars in the telescopic field.
Totally he secured 3400 gauges. His studies
showed that in the region of the Galaxy the
stars were much more numerous than near the
galactic poles. Sometimes he saw as many as
588 stars in a telescopic field, at other times
only 2. He remarked that he had " often
known more than 50,000 pass before his sight
within an hour." Assuming that the stars were,
on the average, of about the same size, and
scattered through space with some approach to
uniformity, Herschel was unable to compute the
extent to which his telescope penetrated into
space ; and, assuming that the Universe was
finite and that his " gauging- telescope " was
sufficiently powerful to completely resolve the
Milky Way, he was enabled to sketch the shape
and extent of the Universe.

Thus Herschel concluded that the Universe
extended in the direction of the Galaxy to 850
times the mean distance of stars of the first
magnitude. In the direction of the galactic
poles the thickness was only 155 times the dis-
tance of stars of the same magnitude. Herschel
was thus enabled to sketch the probable form
of the Universe, which he regarded as cloven at
one of its extremities, the cleft being represented

HERSCHEL THE DISCOVERER. 37

by the famous gap in the Milky Way. The
Universe was, in fact, supposed to be a cloven
disc, and the Milky Way was merely a vastly
extended portion of it and not a region of
actual clustering. On this theory the clusters
and nebulae were supposed to be galaxies ex-
ternal to the Universe. Even in 1785, how-
ever, Herschel believed that there were regions
in the Milky Way where the stars were more
closely clustered than others. " It would not
be difficult," he wrote in 1785, "to point out
two or three hundred gathering clusters in our
system."

Strange to say, Herschel's original ideas re-
garding the Universe were accepted for many
years by astronomical writers. Arago accepted
Herschel's original theory, unaware that he had
in reality abandoned it, and he was followed by
a host of French and English writers who did
not take the trouble to read each of Herschel's
papers, merely quoting that of 1785, and believ-
ing that it represented his final ideas on the
subject. Even Sir John Herschel seems to have
been unaware that his father gave up the disc
theory of the Universe. The famous German
astronomer, Wilhelm Struve, after an exhaustive
study of Herschel's papers, was enabled to prove
in 1847 that the theory had been abandoned

38 A CENTURY'S PROGRESS IN ASTRONOMY.

by Herschel ; and in England the late R. A.
Proctor independently demonstrated the same
thing. Meanwhile, supposing Herschel had not
given up his theory, it would be quite untenable.
After considering the fact that the brighter stars,
down to the ninth magnitude, aggregate on the
Milky Way, Mr Gore says: "As the stars are
by hypothesis supposed to be uniformly dis-
tributed throughout every part of the disc, and
as the limiting circles for stars to the eighth and
ninth magnitudes fall well within the thickness
of the disc, there is no reason why stars of these
magnitudes should not be quite as numerous in
the direction of the galactic poles as in that of
the Milky Way itself. We see, therefore, that
the disc theory fails to represent the observed
facts, and that Struve and Proctor were amply
justified in their opinion that the theory is
wholly untenable, and should be abandoned."

The observations made by Herschel himself
eventually proved fatal to the disc theory — a
hypothesis which he had all along held very
lightly. His ideas about subordinate clusters
within the Milky Way were soon confirmed, and
though in 1799 he still adhered to the disc
theory, he wrote in 1802, "I am now convinced,
by a long inspection and continued examination
of it, that the Milky Way itself consists of stars

HERSCHEL THE DISCOVERER. 39

very differently scattered from those which are
immediately about us. This immense starry
aggregation is by no means uniform. The stars
of which it is composed are very unequally
scattered" — a conclusion quite opposed to the
disc theory, where the Milky Way was sup-
posed to be merely an extended portion of the
Universe.

In 1811 Herschel wrote as follows: "I must
freely confess that by continuing my sweeps of
the heavens, my opinion of the arrangement of
the stars, and their magnitudes, and some other
particulars, has undergone a gradual change ;
and, indeed, when the novelty of the subject is
considered we cannot be surprised that many
things formerly taken for granted should on
examination prove to be different from what
they were generally but incautiously supposed
to be. For instance, an equal scattering of the
stars may be admitted in certain calculations ;
but when we examine the Milky Way, or the
closely compressed clusters of stars, of which
my catalogues have recorded so many instances,
this supposed equality of scattering must be
given up."

This was the virtual abandonment of the disc
theory. Six years later Herschel announced
that in six cases he had failed to resolve the

40 A CENTURY'S PROGRESS IN ASTRONOMY.

Milky Way, stating that his telescopes could
not fathom it. This was the abandonment of his
second assumption — namely, that his telescope
was sufficiently powerful to penetrate to the
limits of the Universe. Yet he still thought
that some of the star-clusters might be external
galaxies, although he could not even dogmati-
cally assert our Universe to be limited. In an
error of translation, Struve left the impression
that Herschel believed our Universe to be un-
fathomable or infinite, and was obliged to devise
a most artificial theory of the extinction of light
to account for the fact that the sky did not
shine with the brilliance of the Sun, which it
would do were the stars infinite in number. Of
course, Herschel did not actually believe the
Universe to be infinite, and, had he lived, he
would probably have shown that all the star-
clusters which we see are included within the
bounds of our finite Galaxy.

In 1814 Herschel was "still engaged in a
series of observations for ascertaining a scale
whereby the extent of the Universe, as far as it
is possible for us to penetrate into space, may be
fathomed." In 1817 he described another method
of star-gauging, which Arago and other writers
have confused with that which he devised in
1785. The two methods, however, were quite

HERSCHEL THE DISCOVERER. 41

distinct from each other. In the first system, one
telescope was used on different regions of the
heavens ; whereas in the second method, various
telescopes were used on identical regions. The
principle was that the telescopic power necessary
to resolve groups of stars indicates the distance
at which the stars of the groups lie. This,
however, also assumed an equal distribution of
stars, and as the late Mr Proctor says, "I
conceive that no question can exist that the
principle is unsound, and that Herschel would
himself have abandoned it had he tested it
earlier in his observing career. ... In ap-
plying it, Sir W. Herschel found regions of
the heavens very limited in extent, where the
brighter stars (clustered like the fainter) were
easily resolved with low powers, but where his
largest telescopes could not resolve the faintest.
These regions, if the principle were true, must
be long, spike-shaped star groups, whose length
is directed exactly towards the astronomer on
Earth, — an utterly incredible arrangement."

Herschel, at the time of his death, left un-
solved the problem of the construction of the
heavens. It is still unsolved, and will doubtless
remain so until astronomers know more about
the distances and motions of the stars. His
last observation of the Galaxy showed that even

42 A CENTURY'S PROGRESS IN ASTRONOMY.

with his 40-foot reflector he could not fathom
it. Consequently, as we have mentioned, Struve
and his successors regarded the Universe as
infinite — a theory which has now received its
death-blow. Herschel was undoubtedly correct
when he stated his belief in a limited Universe.
Herschel's star-gauges, and those of his son,
still remain of immense value to astronomers in
any discussion of the construction of the heavens.
Thus, although they failed to reveal to Herschel
the structure of the Universe, they have been
of much use to his successors. Herschel's discus-
sion of the supreme problem — the ultimate object
of his observations — constitutes one of the most
interesting chapters in the history of science, and
marks a new era in human thought. In the
words of Miss Clerke : " One cannot reflect with-
out amazement that the special life -task set
himself by this struggling musician — originally
a penniless deserter from the Hanoverian Guard
— was nothing less than to search out the
' construction of the heavens.' He did not
accomplish it, for that was impossible ; but he
never relinquished, and, in grappling with it,
laid deep and sure the foundations of sidereal

science."

CHAPTER III.

THE SUN.

FOUR years after the death of Herschel, an
apothecary in the little German town of Dessau
procured a small telescope, with which he began
to observe the Sun. The name of this apothe-
cary was Samuel Heinrich Schwabe (1789-1875).
In 1826 he commenced to observe the spots on
the Sun's disc, counting them from day to day,
more for self- amusement than from any hope of
discovery ; for previous astronomers had agreed
that no law regulated the number of the sun-
spots. Every clear day Schwabe pointed his
telescope at the Sun and took his record of the
spots ; this he continued for forty-three years,
until within a few years of his death on
April 11, 1875. As early as 1843 Schwabe
hinted that a possible period of ten years regu-
lated the distribution of the spots on the Sun,
but no attention was given to his idea. In
1851, however, the result of his twenty -six

44 A CENTURY'S PROGRESS IN ASTRONOMY.

years of observation was published in Hum-
boldt's ' Cosmos,' and Schwabe was able to show
that the spots increased and decreased in a period
of about ten years. Astronomers at once recog-
nised the importance of Schwabe's work, and
in 1857 he was rewarded by the Gold Medal
of the Royal Astronomical Society of London.

Rudolf Wolf (1813-1892) of the Zurich Ob-
servatory now undertook to search through the
records of sun-spot observation, from the days
of Galileo and Scheiner, to find traces of the
solar cycle discovered by Schwabe. He was
successful, and was enabled to correct Schwabe's
estimate of the length of the period, fixing it as
on the average 11 '11 years. Additional interest,
however, was given to Schwabe's and Wolf's
investigations by the remarkable discoveries
which followed. In September 1851 John
Lamont (1805-1879), a Scottish astronomer, —
born at Braemar in Aberdeenshire, but employed
as director of the Munich Observatory, — after
searching through the magnetic records collected
at Gottingen and Munich, discovered that the
magnetic variations indicated a period of 10^-
years. Soon after this Sir Edward Sabine
(1788-1883), the English physicist, from a dis-
cussion of an entirely different set of obser-
vations, independently demonstrated the same

THE SUN. 45

thing, proving conclusively that once in about
ten years magnetic disturbances reached their
height of violence ; and Sabine was not slow
to notice the correspondence between the mag-
netic period and the sun-spot period. In the
same year (1852) Wolf and Alfred Gautier
(1793-1881) independently made the same dis-
covery, which had thus been made by four
separate investigators.

In the same year an English amateur as-
tronomer, Richard Christopher Carrington (1826-
1875), commenced a series of solar observations
which led to some remarkable discoveries. From
observations on the spots, Carrington discovered
that while the Sun's rotation was performed in
25 days at the equator, it was protracted to
27|- days midway between the equator and the
poles. In 1858 Carrington demonstrated the
fact that spots are scarce in the vicinity of the
solar equator, but are confined to two zones
on either side, becoming scarce again at thirty-
five degrees north or south of the equator.
Contemporary with Carrington was Friedrich
Wilhelm Gustav Sparer (1822-1895), who was
born in Berlin in 1822 and died at Giessen,
July 7, 1895. He commenced his solar ob-
servations about the same time as Carrington,
and independently discovered the Sun's equatorial

46 A CENTURY'S PROGRESS IN ASTRONOMY.

acceleration. From observations at his little
private observatory at Anclam in Pomerania,
continued at the Astrophysical Observatory in
Potsdam, Sporer demonstrated a remarkable law
regarding sun-spots. This law is thus described
by a well-known astronomer : " The disturbance
which produces the spots of a given sun-spot
period first manifests itself in two belts about
thirty degrees north and south of the Sun's
equator. These belts then draw in toward the
equator, and the sun-spot maximum occurs when
their latitude is about sixteen degrees ; while
the disturbance gradually and finally dies out
at a latitude of eight or ten degrees. Two or
three years before this disappearance, however,
two new zones of disturbance show themselves.
Thus, at the sun-spot minimum there are four
well-marked spot-belts, — two near the equator,
due to the expiring disturbance, and two in
high latitudes, due to the newly beginning
outbreak." These remarkable discoveries, which
resulted from the investigations of Schwabe,
Carrington, and Sporer, are a brilliant example
of what may be done by amateurs in astronomy.
At the time when Carrington and Sporer were
pursuing these researches, the spectroscope came
into use as an astronomical instrument, and since
1859 solar astronomy has been almost entirely

THE SUN. 47

spectroscopic. Before we can rightly understand
the principles of spectroscopic astronomy, we must
go back to the life and work of its founder —
Joseph von Fraunhofer.

The son of a poor glazier, Joseph von Fraun-
hofer was born on March 6, 1787, at Straubing,
in Bavaria. His father and mother having died
when their son was quite young, the boy, on
account of his poverty, was apprenticed to a
looking - glass manufacturer in Munich named
Weichselberger, who acted tyrannically, keeping
him all day at hard work. Still the lad borrowed
some old books, and spent his nights in study.
Young Fraunhofer lodged in an old tenement
in Munich, which on July 21, 1801, collapsed,
burying in its ruins its occupants. All were
killed but Fraunhofer, who, though seriously
injured, was dug out from the ruins four hours
later. The distressing accident was witnessed
by Prince Maximilian Joseph, Elector of Bavaria.
He became interested in Fraunhofer, and pre-
sented him with a sum of money. Of this he
made good use. He was already interested in
optics, and he bought some books on that sub-
ject, as well as a glass-polishing machine. The
remainder of the money served to procure his re-
lease from his tyrannical master, Weichselberger.

Fraunhofer became acquainted with prominent

48 A CENTURY'S PROGRESS IN ASTRONOMY.

scientists at Munich, who provided him with
books on optics and mathematics. Meanwhile the
young optician occupied his time in shaping
and finishing lenses. In 1806 he entered the
optical department of the Optical and Physical
Institute of Munich, and the following year,
when only twenty years of age, was appointed
to the chief post in that department. In 1814
he commenced his investigations with the prism,
which have made his name famous.

Newton had found that, in passing through a
prism, white light is dispersed into its primary
colours, making up the band of coloured light
known as the solar spectrum. But he failed
to recognise the existence of dark lines in the
spectrum. Casually seen in 1802 by William
Hyde Wollaston (1786-1828), an English physicist,
these lines were first thoroughly examined by
Fraunhofer. Allowing light from the Sun to
pass through a prism attached to the telescope,
he was amazed to find several dark lines in
the spectrum. By the year 1814 he had de-
tected no less than 300 or 400 of these lines.
Fraunhofer named the more prominent lines by
the letters of the alphabet, from A in the red
to H in the violet. They are now known as
the Fraunhofer lines. At first he was much
perplexed regarding the nature of the dark lines.

THE SUN. 49

He suspected that they might be an optical
effect, depending on the quality of the glass
used, and he tried different prisms, but the
lines were still to be seen. Then he turned
his prism to bright clouds to see if they were
visible in reflected sunlight, and he found that
they were. He examined the Moon and again
perceived them, as moonlight is merely reflected
sunlight ; and they were also conspicuous in the
spectra of the planets. It was thus proved that
these lines were characteristic of sunlight, whether
direct or reflected. It was, however, still possible
that they might be caused by the passage of the
rays of light from the celestial bodies through the
Earth's atmosphere. In order to test this theory,
Fraunhofer examined the spectra of the brighter
stars. He found that the lines visible in the
solar spectrum were not to be seen in the spectra
of the stars, thus proving that the lines were not
merely an atmospheric effect. Each star, Fraun-
hofer observed, had a different spectrum from
both the Sun and from other stars. These
spectra were also characterised by numerous
dark lines, much fainter than those in the solar
spectrum.

Although he ascertained the existence of the
dark lines in the Sun's spectrum, Fraunhofer
never really found out what they represented.

50 A CENTURY'S PROGRESS IN ASTRONOMY.

As Miss Giberne expresses it, " Although he now
saw the lines he could not understand them :
he could not read what they said. They spoke
to him indeed about the Sun, but they spoke
to him in a foreign language, the key to which
he did not possess." However, he expressed the
belief that the pair of lines in the solar spectrum,
which he marked D, coincided with the pair
of bright lines emitted by incandescent sodium.
Although he doubtless suspected that the lines
conveyed intelligence regarding the elements in
the Sun, he never was able properly to decipher
their meaning. Had he lived, he would prob-
ably have made the great discovery; but these
investigations were cut short by his sudden
and untimely death on June 7, 1826.

After the death of Fraunhofer, very little
was done to forward the study of spectrum
analysis. Investigations in this branch of
research were made, however, by Sir John
Herschel (1792-1871), William Allen Miller
(1817-1870), Sir David Brewster (1781-1868), and
others. Two famous men of science had partly
discovered the secret. These were Sir George
Stokes (1819-1903), of Cambridge, and Anders
John Angstrom (1812-1872) of Upsala. Of
Angstrom's work, published in 1853, it has
been said that it would " have obtained a high

THE SUN. 51

celebrity if it had appeared in French, English,
or German, instead of Swedish."

It was not until 1859 that the principles of
spectrum analysis were fully enunciated by
Gustav Robert Kirchhoff (1824-1887), and his
colleague in the University of Heidelberg, Robert
Wilhelm Bunsen (1811-1899). Kirchhoff demon-
strated that a luminous solid or liquid gives a
continuous spectrum, and a gaseous substance a
spectrum of bright lines. In the words of Miss
Clerke, " Substances of every kind are opaque
to the precise rays which they emit at the
same temperature. That is to say, they stop
the kinds of light or heat which they are then
actually in a condition to radiate. . . . This
principle is fundamental to solar chemistry. It
gives the key to the hieroglyphics of the Fraun-
hofer lines. The identical characters which are
written bright in terrestrial spectra are written
dark in the unrolled sheaf of sun-rays." Kirch -
hoff made several determinations of the sub-
stances in the Sun, proving the existence of
sodium, iron, calcium, magnesium, nickel, barium,
copper, and zinc. His great map of the solar
spectrum was published by the Berlin Academy
in 1860, and represented an enormous amount
of labour. It was succeeded by another map
by Angstrom, published in 1868. But both of

52 A CENTURY'S PROGRESS IN ASTRONOMY.

these maps have been recently superseded by
the investigations of Sir Joseph Norman Lockyer
(born 1836), and of the American physicist,
Henry Augustus Rowland (1848-1901). Row-
land largely increased our knowledge of the
elements in the solar atmosphere.

The spectroscope had become, by 1868, a
recognised instrument of astronomical research,
and in that year it was applied during the
famous total eclipse, visible in India. There
were many eclipse problems, arising from the
observations made by the eclipse expeditions
of 1842, 1851, and 1860. The eclipse of 1851
had finally proved that the red flames seen
surrounding the Sun during total eclipses be-
longed to the Sun, and not to the Moon, as
many astronomers had believed. At the eclipse
of 1860, visible in Spain, the Italian astronomer,
Angelo Secchi (1818-1878), and the Englishman,
Warren De la Rue (1815-1889), secured photo-
graphs of the solar prominences. The problem of
1868 was the constitution of these prominences.

Pierre Jules Cesar Janssen, born in Paris in
1824, was stationed at Guntoor, in India, to
observe the eclipse. He succeeded in observing
the spectrum of the prominences during the
progress of totality, and found it to be one of
bright lines, proving the gaseous nature of the

THE SUN. 53

sun-flames. During the progress of the eclipse,
Janssen was specially struck by the brilliancy
of the bright lines, and it occurred to him that
the prominence -spectrum could be observed in
full daylight, if sufficient dispersive power was
used to enfeeble the ordinary continuous spec-
trum. At ten o'clock on the following morning,
August 19, 1868, Janssen applied his spectro-
scope to the sun, and observed the prominence-
spectrum. After a month's observation in India,
he sent to the French Academy an account of
his success. A short time, however, before his
report arrived, the Academy had received a
similar one from Lockyer, who had independently
made the same discovery. Two years previously,
in 1866, the new method had occurred to him,
but his spectroscope was not powerful enough ;
and although he ordered a more powerful one
at once, it was not until October 16, 1868,
that he had the instrument in his hands. Four
days later he observed the prominence-spectrum
in full daylight.

The next advance in the study of the promi-
nences was announced in 1869. Janssen and
Lockyer had shown astronomers how to observe
the spectrum of the prominences ; but the re-
searches of other two famous astronomers enabled
observers to see the forms of the prominences.

54 A CENTURY'S PROGRESS IN ASTRONOMY.

These two men were William Huggins (born
1824) and Johann Carl Friedrich Zollner. The
latter astronomer, born in Leipzig in 1834, was
one of the most successful students of the solar
prominences. He was Professor of Astrophysics
in the University of Leipzig, a position which
he filled with success until his untimely death
on April 25, 1882. Independently of Huggins,
he found that by opening the slit of the
spectroscope wider, the forms of the promi-
nences themselves could be seen. The study
of the prominences was at once taken up by
the most famous solar observers : these were
Huggins and Lockyer in England, Sporer and
Zollner in Germany, Janssen in France, Secchi,
Respighi, and Tacchini in Italy, Young in
America. To Charles Augustus Young (born
1834) we owe the careful study of individual
prominences. On September 7, 1871, he ob-
served the most gigantic outburst on the sun
ever witnessed, fragments of an exploded promi-
nence reaching a height of 100,000 miles : Young,
also, made the first attempt to photograph the
prominences.

To the Italian school of astronomers, however,
we owe the persistent and systematic study
of the prominences. Among them the three
greatest names are Angelo Secchi (1818-1878),

THE SUN. 55

Lorenzo Respighi (1824-1889), emdPietro Tacchini
(1838-1905). After the death of Secchi, the
recognised head of spectroscopy in Italy was
Pietro Tacchini. Born at Modena in 1838, he
was appointed director at Modena in 1859,
assistant at Palermo in 1863, and director at
Rome in 1879. In 1870 he commenced to take
daily observations of the prominences, noting
their sizes, forms, and distribution, and these
observations were continued for thirty-one years,
until within four years of Tacchini's death, which
took place on March 24, 1905. Tacchini did
for the study of the prominences what Schwabe
did for the spots. The Italian spectroscopists
found that the prominences increased and de-
creased every eleven years in harmony with
the spots. Tacchini demonstrated that the
streamers of the solar corona originate in
regions where the prominences are most numer-
ous, and that the shape of the corona, on the
whole, varies in sympathy with the prominences.
The researches of Lockyer indicated that the
prominences originated in a shallow gaseous
atmosphere which he termed the chromosphere.
Formerly astronomers had to observe only
isolated prominences, but in 1892 an American
astronomer, George Ellery Hale (born 1868),
formerly director of the Yerkes Observatory,

56 A CENTURY'S PROGRESS IN ASTRONOMY.

and now director of the Solar Observatory in
California, succeeded in photographing, by an
ingenious process, the whole of the chromo-
sphere, prominences, and faculse visible on the
solar surface.

Another solar envelope was discovered in 1870
by Dr Charles Augustus Young, who from 1866
to 1877 directed the Observatory at Dartmouth,
New Hampshire, and from 1877 to 1905, that
at Princeton, New Jersey. During the eclipse
of December 22, 1870, Young was stationed at
Tenez de Frontena, Spain. As the solar crescent
grew apparently thinner before the disc of the
Moon, " the dark lines of the spectrum," he says,
" and the spectrum itself gradually faded away,
until all at once, as suddenly as a bursting rocket
shoots out its stars, the whole field of view was
filled with bright lines, more numerous than one
could count. The phenomenon was so sudden,
so unexpected, and so wonderfully beautiful, as
to force an involuntary exclamation." The phe-
nomenon was observed for two seconds, and
the impression was left on the astronomer that
a bright line had taken the place of every
dark one in the solar spectrum, the spectrum
being completely reversed. Hence the name
which was given to the hypothetical envelope —
"the reversing layer." For long the existence

THE SUN. 57

of the reversing layer was disputed by numerous
astronomers. In 1896 photographs taken during
the solar eclipse of that year finally demon-
strated the existence of the "flash spectrum"
as seen by Young.

The last of the solar appendages, the corona,
can only be seen during total eclipses. The
researches of Young and Janssen indicate that
it is partly gaseous and partly meteoric in its
constitution ; and various photographs, taken at
the eclipses since 1870, have demonstrated its
variation in shape, which is in harmony with
the eleven-year period. Several attempts have
been made to observe the corona without an
eclipse. In 1882 Huggins made the attempt,
but failed, and Hale, with his photographic
process, had no better success. More recently,
in 1904, a Russian astronomer, Alexis Hansky,
observing from the top of Mont Blanc, secured
some photographs on which he believes the
corona is represented, but so far his obser-
vations have not been confirmed by other
astronomers.

The application of the spectroscope to the
motions on the solar surface is perhaps one
of the most wonderful triumphs in astronomical
science. In 1842 Christian Doppler (1803-1853),
Professor of Mathematics at Prague, had ex-

58 A CENTURY'S PROGRESS IN ASTRONOMY.

pressed the view that the colour of a luminous
body must be changed by its motion of approach
or recession. It was obvious to Doppler that
if the body was approaching, a larger number
of light waves must be entering the eye of
the observer than if it were retreating. Miss
Clerke thus illustrates Doppler's principle :
"Suppose shots to be fired at a target at
fixed intervals of time. If the marksman
advances, say, twenty paces between each dis-
charge of his rifle, it is evident that the
shots will fall faster on the target than if
he stood still ; if, on the contrary, he retires
by the same amount, they will strike at cor-
respondingly longer intervals." It occurred to
various astronomers that it would be possible
to measure cyclones and hurricanes in the Sun,
not by change of colour in the spectrum, but
by the shifting of the lines ; and in 1870 this
was successfully done by Lockyer. In the next
few years efforts to measure the solar rotation
were made by Young, Zollner, and others, who
succeeded in measuring the displacement of the
lines, but not the time of rotation. This was
reserved for the famous Swedish astronomer,
Duner.

Nils Christopher Duner > born in 1839 in
Scania, was employed as an assistant at Lund

THE SUN, 59

Observatory from 1858 to 1888, when he was
appointed director of the Observatory at Upsala.
In that year he commenced a study of the solar
rotation, measuring it by means of Doppler's
principle. He confirmed the telescopic work
of Carrington and Sporer on the equatorial
acceleration, and measured the displacement up
to within fifteen degrees of the poles. He
brought out the surprising fact that the rota-
tion period of the Sun is there protracted to
38^ days. These remarkable researches were
published in 1891.

In 1873 the Astronomer -Royal of England
commenced at Greenwich Observatory to photo-
graph the Sun daily. This work has been
carried on there by Edward Walter Maunder
(born 1851), and Greenwich Observatory pos-
sesses a photographic record of sun-spots. At
the Meudon Astrophysical Observatory, near
Paris, Janssen has, since 1876, secured photo-
graphs of the solar surface, which were com-
prised in a great atlas, published by him in
January 1904. These photographs have re-
vealed a remarkable phenomenon — the " reseau
photospherique," the distribution over the solar
surface of blurred patches of light, which
Janssen considers are inherent in the Sun.
The Greenwich records of sun - spots and of

60 A CENTURY'S PROGRESS IN ASTRONOMY.

magnetic disturbances have been made use of
by Maunder in his remarkable studies, prom-
ulgated in 1904, of the connection between
sun-spots and terrestrial magnetism. Maunder
finds that on the average magnetic storms are
dependent on the presence of sun-spots, and on
the size of the spot. The magnetic action, he
finds, does not radiate equally in all directions
from the sun-spots, but along definite and
restricted lines.

Herschel's hypothesis of a dark and cool
globe beneath the solar photosphere was seen
to be untenable after the introduction of the
spectroscope. The first important theory as to
the solar constitution was that advanced in
1865 by the French astronomer, Herve Faye
(1814-1902). Numerous other theories were
afterwards advanced by Secchi, Zollner, Young,
and others, but a complete description of the
various developments in solar theorising cannot
be given here. There is no complete " theory "
of the exact constitution of every part of the
Sun, but the unpretentious "Views of Professor
Young on the Constitution of the Sun," which
appeared in April 1904 in 'Popular Astronomy/
represent the latest ideas of the foremost solar
investigator. Professor Young regards the re-
versing layer and the chromosphere as "simply

THE SUN. 61

the uncondensed vapours and gases which form
the atmosphere in which the clouds of the
photosphere are suspended." He says that the
contraction theory of Helmholtz, — explained in
another chapter, — advanced to explain the main-
tenance of the Sun's heat, is true so far as it
goes ; but that it is all the truth is now made
doubtful by the discovery of radium, which
" suggests that other powerful sources of energy
may co-operate with the mechanical in main-
taining the Sun's heat."

The important question of the distance of the
Sun was thoroughly investigated in 1824 by
Johann Franz Encke (1791-1865), then of See-
berg, near Gotha, who, from a discussion of the
transits of Venus in 1761 and 1769, found a
parallax of 8"*371, corresponding to a mean
distance of 95,000,000 miles. This value was
accepted for thirty years, until Peter Andreas
Hansen (1795-1874), in 1854, and Urban Jean
Joseph Le Verrier (1811-1877), in 1858, found
from mathematical investigations that the dis-
tance indicated was too great. Preparations
were accordingly made for the observation of
the transits of Venus, which took place respect-
ively on December 8, 1874, and December 6,
1882. On the first occasion many expeditions
were sent to view the transit, consisting of

62 A CENTURY'S PROGRESS IN ASTRONOMY.

French, German, American, English, Scottish,
Italian, Russian, and Dutch astronomers, and
it was hoped that the solar parallax would be
accurately measured once for all. However, the
transit, although favoured with good weather,
was not successful, owing to the difficulty of
making exact measurements, by reason of the
illumination and refraction in the atmosphere
of Venus. Accordingly the values deduced for
the parallax were far from unanimous. The
transit of 1882 was not observed so extensively,
as astronomers had found the transit of Venus
to be by no means the best method. In 1877
Sir David Gill (born 1843), the great Scottish
astronomer, determined the solar parallax suc-
cessfully from measures of the parallax of Mars
in opposition. His value was 8 "'7 8, correspond-
ing to 93,080,000 miles. Some years previous
to this Johann Gottfried Galle (born 1812), the
German astronomer, had, from measurements
of the parallax of the asteroid Flora, deduced
a solar parallax of 8" '8 7. Gill's work at the
Cape in 1888, on the Asteroids, was successful
in giving a more accurate value than the transit
of Venus: in 1900 and 1901 measures of the
parallax of the asteroid Eros, the nearest minor
planet, were made by many different observa-
tories, and agree with the other results. The

THE SUN. 63

values which have been derived from the velocity
of light, and from the constant of aberration,
are fairly in agreement with those derived from
direct measurement. On the whole, the most
probable value of the parallax is about 8" "8,
indicating a mean distance of about 92,700,000
miles, with a " probable error" of about 150,000
miles.

What a different picture the sun presents to
us at the beginning of the twentieth century
from that which it presented to Herschel and
his contemporaries at the beginning of the
nineteenth ! To Herschel, the Sun was a cool
dark globe, surrounded by a luminous atmosphere.
As the outcome of the researches and discoveries
outlined in this chapter, the Sun is now seen
to be a vast central world, which is over a
million times larger than the Earth. In the
words of an able writer, "It is most prob-
ably a world of gases, where most of the metals
and metallic bases that we know exist only as
vapours, even at the Sun's surface, hotter
than any furnace on earth, and getting a still
fiercer heat for every mile of descent lower.
Of that heat in the Sun's interior we can
form no conception. The pressure within the
Sun is equally inconceivable. A cannon - ball
weighing 100 Ib. on earth would weigh 2700

64 A CENTURY'S PROGRESS IN ASTRONOMY.

on the Sun. Thus a mighty conflict goes on
unceasingly between imprisoned and expand-
ing gases and vapours struggling to burst out,
and massive pressures holding them down. For
reasons we cannot fully understand, no equi-
librium is reached. For millions of years up-
rushes and down-rushes of the white-hot materials
have been proceeding on that bright photosphere
which gives us light, and looks a picture of
calm and quiescence. Above that is a com-
paratively thin rose-coloured layer, the chromo-
sphere, agitated with fiery ' prominences/ and
outside all these the coronal glory — all alike
pointing to immeasurable activities."

The following remark of Professor Newcomb
shows our inability to realise the solar activity.
" Suppose," he says, " every foot of space in a
whole country covered with 13 -inch cannon,
all pointed upward, and all discharged at once.
The result would compare with what is going
on inside the photosphere about as much as
a boy's popgun compares with the cannon."

CHAPTER IV.

THE MOON.

IT is somewhat remarkable that the one celestial
body which Herschel neglected was our satellite,
the Moon; and it is also remarkable that the
Moon was for many years the chief object of
study of his contemporary astronomer, Johann
Hieronymus Schroter (1745-1816). Born at
Erfurt, near Hanover, on August 30, 1745,
Johann Hieronymus Schroter was originally
intended for the study of law, for which he
was sent to the University of Gottingen. At
the same time he studied mathematics, and
particularly astronomy, under the mathematician,
Kaestner of Gottingen. Deeply interested in
music, he became acquainted with the Herschel
family, and, inspired by William Herschel's ex-
ample, determined to study the heavens. In
1779 he became the possessor of a small achro-
matic refractor, and commenced to observe the
Sun and Moon. In 1778 he entered the legal

E

66 A CENTURY'S PROGRESS IN ASTRONOMY.

profession at Hanover, and four years later he
was appointed " oberamtmann " or Chief Magis-
trate of Lilienthal — "the Vale of Lilies" — in
the Duchy of Bremen. At Lilienthal Schroter
erected a small observatory, and acquired in 1785
one of Herschel's 7 -foot reflectors. In 1792
the astronomer superintended the construction
of a 13-foot reflector, made by Schrader of Kiel,
who transferred his workshop to Lilienthal. With
these instruments the great work of Schroter was
accomplished.

Schroter directed his powers of observation to
the study of the Moon. He originated the study
of the surface of the Moon, and founded the
branch of astronomy known as selenography,
or the study of the Moon's surface. The
foundations of this branch were laid in 1791
with the publication of Schroter's ' Seleno-
topographische Fragmented The astronomer
determined to make a comparative study of
the surface of our satellite, and before 1801
discovered eleven "rills" or clefts on the
Moon's surface, and recognised a large number
of craters. He likewise believed that he had
seen a lunar atmosphere, an observation of
which was made by him in February 1792.
Schroter seems never to have doubted what
Herschel and his contemporaries believed — that

THE MOON. 67

the Moon was a living world with volcanoes in
active eruption, surrounded by an atmosphere,
and inhabited by beings like ourselves. Unfor-
tunately, Schroter was not good at making
drawings of what he saw ; nevertheless, he
accomplished a vast amount of work. In the
little observatory at Lilienthal the foundations
were laid of the comparative study of the surface
of the Moon.

But these observations were destined to be
rudely interrupted. In 1810 Hanover was occu-
pied by the invading troops of Napoleon, and
Schroter lost his appointment as Chief Magistrate
of Lilienthal, and also his income. But there
was worse to follow. On April 20, 1813, three
years after, the French, under Vandamme, with
that cruelty which seems to belong to warfare,
occupied Lilienthal, and set fire to the little
village. A few days later the French soldiers
entered the observatory and burned it to the
ground. All Schroter's precious observations,
accumulated after thirty-four years' labour, were
destroyed with a few exceptions, the observa-
tions on Mars narrowly escaping the conflagra-
tion. Unable to forget the destruction of his
observatory, and without the means to repair
the loss, he lived only three years after the
disaster. He died on August 29, 1816, "leaving

68 A CENTUKY'S PROGRESS IN ASTRONOMY.

behind him," says Mr Arthur Mee, "an im-
perishable record, and a noble example to ob-
servers of all time."

Wilhelm Gotthelf Lohrmann, a land-surveyor of
Dresden, continued the observations of Schroter,
and in 1824 published four of the twenty-five
proposed sections of a large lunar chart. In
1827, however, his sight began to fail, and he
was obliged to abandon his intention. But a
successor had already appeared on the scene.
Johann Heinrich von Madler (1794-1874) was
born in Berlin in 1794, and, after a severe
struggle to earn a living, entered the University
of Berlin in 1817. In 1824 he became acquainted
with Wilhelm Beer (1797-1850), a wealthy
banker, who had come to him for instruction
in astronomy, and who erected in 1829 an ob-
servatory near his villa in Berlin, where pupil
and tutor pursued their studies.

In 1830 Madler, with Beer's assistance, com-
menced a great trigonometrical survey of the
surface of the Moon. The observations of
Beer and Madler were made with no larger in-
strument than a 3f-inch refractor. They ascer-
tained the positions of 919 lunar spots, and
measured the height of 1095 mountains. Their
great chart of the Moon — which was afterwards
followed by a smaller one — was issued in four

THE MOON. 69

parts during 1834-36. "The amount of detail,"
wrote Proctor, " is remarkable, and the labour
actually bestowed upon the work will appear
incredible." The chart has neither been revised
nor superseded, and it remains to this day one
of the standard works on the subject.

The chart was succeeded in 1837 by a descrip-
tive volume entitled ' Der Mond.' In this work
Beer and Madler did much for the progress of
lunar astronomy. Their observations led to a
change of opinion regarding our satellite's physical
condition. Herschel, Schroter, Olbers, and other
astronomers seem to have considered the Moon
a living world. Madler declared that it was a
dead world. He believed it to be destitute of
life of any kind, and the changes observed by
Schroter and other observers were put down
as illusions. 'Der Mond' was the end of
Madler's work in lunar astronomy, for, receiving
an appointment at Dorpat, he went there in
1846, and retained his post until within a few
years of his death, which took place at Hanover
on March 14, 1874.

Madler's successor in the field of lunar as-
tronomy was Johann Friedrich Julius Schmidt
(1825-1884), who was born at Eutin in Liibeck
in 1825. At a very early age he gave indica-
tions of a taste for astronomy. Fortunately his

70 A CENTURY'S PROGRESS IN ASTRONOMY.

father possessed a small hand telescope, with
which young Schmidt commenced his lunar
studies. Appointed assistant at Bonn and
Olmtitz and director at Athens successively,
he kept up his persistent study of the surface
of the Moon for over forty years. In 1839,
when fourteen years of age, he began the valu-
able series of observations which were destined
to form the basis of his great chart of the sur-
face of the Moon. Between 1853 and 1858,
when employed at Olmtitz, Schmidt made and
calculated no fewer than 4000 micrometrical
measures of the altitudes of lunar mountains.
Before 1866 Schmidt had found no fewer than
278 " rills," and his discoveries were the means of
augmenting the number of these curious objects
to nearly a thousand.

In a word, it may be said that Schmidt drew
out a lunar geography, and the result of his
labours, together with those of Schroter and
Madler, is that in a sense we now know the
features of the Moon better than those of the
Earth. For instance, astronomers see the whole
surface of the Moon spread before their eyes,
while geographers can never have a similar
view of the terrestrial features : we have never
seen the poles of the Earth, while the lunar
poles are well known to astronomers. For

THE MOON. 71

twenty years after his appointment at Athens,
Schmidt worked at fixing the positions of lunar
objects, measuring the heights of mountains and
the depths of craters. An idea of his enthusiasm
in constructing his great chart may be gained
from the fact that he made almost a thousand
original sketches.

Madler's dogmatic assertion that the Moon
was entirely a dead world was generally
believed until Schmidt made observations to
the contrary. From 1837 to 1866 the popular
opinion was that our satellite was an absolutely
dead world. Consequently there was little prog-
ress in lunar astronomy during those thirty
years. Although Madler's view was much
nearer the truth than the opinions of his
predecessors, it was also too positive. His
confident assertion, which was received with-
out hesitation, was never questioned until
Schmidt came upon the scene. To Schmidt
the Moon was not entirely dead, and it was
he who brought forward indisputable evidence
as to the existence of changes on its surface.
In October 1866 he announced that the crater
Linne had lost all appearance of such, and
that it had become entirely effaced. Lohrmann
and Madler had observed it under a totally
different aspect, as also had Schmidt himself

72 A CENTURY'S PROGRESS IN ASTRONOMY.

from 1840 to 1843. There was great excite-
ment in the astronomical world on Schmidt's
announcement, and many astronomers denied
the change, although Schmidt's observation was
confirmed by Secchi and Webb. The evidence
in favour of it preponderated, and very few
observers now consider the Moon's surface to
be absolutely changeless.

In 1865 Schmidt had begun to arrange his
observations on the Moon into the form of a
chart. At first he decided to have a chart of
six feet diameter, divided, like that of Madler,
into four sections. But in April 1868, on
making an estimate of the value of such a
chart, he was dissatisfied, and determined to
construct a map of the same size divided into
twenty-five sections instead of four. He began
the work in 1868, and after six years the
great map was completed. After some delay
the German Government undertook to issue the
chart at their expense, and it was published in
1879, after fourteen years of preparation. It
contained no fewer than 30,000 objects, and its
completed diameter was six feet three inches —
more than double the size of any previous
map of the Moon. Indeed, it was probably
the greatest contribution ever made to lunar
astronomy. Schmidt lived only a few years

THE MOON. 73

after the publication of his great chart. He
died at Athens, in his fifty - ninth year,
February 8, 1884.

Schmidt's announcement of the change in the
appearance of Linnd was followed in 1878 by
a statement by Hermann Joseph Klein (born
1842) of Cologne, to the effect that a new
crater had been formed to the north of the
well-known lunar crater, Hyginus. The change
in this case, however, is by no means so certain
as in that of Linne. It will be observed that
the majority of the students of the Moon
were Germans. In England the study was
not taken up until 1864, when a Lunar
Committee of the British Association was ap-
pointed. Some good lunar work was done by
the well-known astronomer, Thomas William
Webb (1807-1885), while the study was pop-
ularised by James Nasmyth (1808-1890), the
famous engineer, who published, in 1874, in
conjunction with James Carpenter of Greenwich
Observatory, a beautifully -illustrated volume
entitled 'The Moon/ This was succeeded, in
1876, by the larger work of Edmund Neison
(now Nevill), Government Astronomer of Natal.
About this time several English astronomers,
devoted to the study of the Moon, formed them-
selves into the Selenographical Society. After

74 A CENTURY'S PROGRESS IN ASTRONOMY.

a few years, however, the society came to an
end, and the enthusiasts formed themselves into
the lunar section of the British Astronomical
Association, on the foundation of that society
in 1890. Chief among those English seleno-
graphers was Thomas Gwyn JElger (1837-1897),
whose observations of the Moon and drawings
of the various craters were of the utmost value.
Two years before his death, in 1895, Elger pub-
lished his important work, * The Moon/ along
with an exhaustive chart of the visible face of
our satellite.

Herschel and Schroter firmly believed in the
existence of a lunar atmosphere, the latter
believing that he had actually observed the
Moon's atmospheric envelope. Early in the
nineteenth century it was soon observed, how-
ever, that on the Moon passing over and occult-
ing stars, these stars disappeared suddenly
behind the Moon's limb, instead of gradually,
as they should have done, had an atmosphere
of any density existed. Accordingly astrono-
mers gave up believing in a lunar atmosphere.
On January 4, 1865, Huggins observed with
his spectroscope the occultation of a small star
in Pisces. There was not the slightest sign of
absorption in a lunar atmosphere ; the entire
spectrum vanished at once.

THE MOON. 75

Lunar photography was introduced as long
ago as 1858 by Lewis Morris Rutherfurd (1816-
1892), the well-known American astronomer ; but
for years very little was done in this matter,
although Rutherfurd secured fairly good photo-
graphs. Rutherfurd, De la Rue, and the older
astronomical photographers took photographs of
the entire Moon, but this plan was abandoned
in favour of what Miss Clerke calls " bit by bit
photography." About 1890 this method was
introduced, and has been followed with success
by Maurice Loewy (born 1833), and his assistant,
Pusiex, at the Paris Observatory ; by Ladislas
Weinek at Prague ; by the astronomers of the
Lick Observatory; and by William Henry Picker-
ing (born 1858), the distinguished astronomer of
Harvard, whose discoveries and investigations
have created quite a new interest in lunar
astronomy. These investigations were com-
menced in 1891 at Arequipa, on the slope of
the Andes, in Peru. An occultation of Jupiter,
witnessed by W. H. Pickering on October 12,
1892, gave support to the view that a very
tenuous lunar atmosphere does exist. In 1900
he established, near Mandeville, Jamaica, a tem-
porary astronomical station, where he obtained
many excellent photographs. Totally he secured
eighty plates. These appeared, as the first com-

76 A CENTURY'S PROGRESS IN ASTRONOMY.

plete photographic lunar atlas ever published, in
his work 'The Moon' (1903), in which he sums
up all his observations since 1891, and concludes
that "the evidence in favour of the idea that
volcanic activity upon the Moon has not yet
ceased is pretty strong, if not fairly conclusive."

Pickering points out that the density of the
lunar atmosphere is not greater than one ten-
thousandth of that at the Earth's surface, and,
under these circumstances, water cannot exist
above freezing-point, which of course brings us
to the subject'of snow. He considers that snow
is observed on the mountain peaks and near the
poles of the Moon, and he believes his conclusion
to be verified by observations on the well-known
crater, Linnd He brings forward evidence of
the probable existence on the Moon of organic
life, pointing out that the difference between
the conditions of the Earth and the Moon is
not so great as that above and below the ocean
on our own planet. He has collected evidence
of the existence of something resembling vege-
tation on the Moon " coming up, flourishing, and
dying, just as vegetation springs and withers
on the Earth."

The first successful attempt to measure the
heating power of moonlight was made in 1846 on
Mount Vesuvius by Melloni, an Italian physicist,

THE MOON. 77

whose results were confirmed four years later by
Zantedeschi, another Italian. The most import-
ant work in this direction was accomplished
by the present Earl of Rosse (born in 1840),
who in the years 1869-72 believed himself to
have measured the lunar heat ; but these con-
clusions were not altogether confirmed by the
observations of Dr Otto Boeddicker (Lord Rosse's
astronomer), during the total lunar eclipse of
October 4, 1884. Further investigations on this
subject were afterwards made by Samuel Pierpont
Langley (1834-1906), of Alleghany, and by his
assistant, Frank Very.

The motion of the Moon and its perturbations
were made the subject of deep study by the
famous Pierre Simon Laplace (1749-1827), the
contemporary of Herschel, and the worthy suc-
cessor of Newton. He devoted much attention
to the secular acceleration of the Moon's mean
motion, a problem which had baffled the greatest
mathematicians. After a profound discussion he
found, in 1787, that the average distance of the
Earth and Moon from the Sun had been slowly
increasing for several centuries, the result being
an increase in the Moon's velocity. In the third
volume of the ' Mecanique Celeste' Laplace
worked out the lunar theory in great detail,
although he calculated no lunar tables. After

78 A CENTURY'S PROGRESS IN ASTRONOMY.

his death the subject was taken up by Charles
Theodore Damoiseau (1768-1846), and the most
important advance was made by Giovanni
Antonio Amadeo Plana (1781-1864), the director
of the Turin Observatory, who published in 1832
a very complete lunar theory. The work of
Plana was followed by that of Peter Andreas
Hansen (1795-1874), whose lunar tables were
used for the Nautical Almanac, and whom
Professor Simon Newcomb considers to be
the greatest master of celestial mechanics since
Laplace. The theory of the Moon's motion was
worked out in detail by the famous astronomer
Charles Eugene Delaunay (1816-1872), who
from 1870 till 1872 occupied the post of
director of the Paris Observatory. Delaunay
was about to work out the lunar tables when,
in 1872, he was accidentally drowned by the cap-
sizing of a pleasure-boat at Cherbourg. The
work accomplished in this direction by Simon
Newcomb (born 1835) is of great importance,
particularly in his correction of Hansen's tables.
John Couch Adams (1819-1892), one of the dis-
coverers of Neptune, while at work on the lunar
theory, had occasion to correct Laplace's sup-
posed solution of the acceleration of the lunar
motion. On going over the calculation Adams
found that several quantities, omitted by Laplace

THE MOON. "79

as unimportant, showed that the Moon has a
minute increase of speed for which the theory
of gravitation will not account, — a conclusion
opposed by Plana, Hansen, and Pontecoulant,
but fully confirmed by Delaunay. Delaunay
suggested in 1865 that the minute apparent
increase was due to the retardation of the
Earth's rotation by tidal friction. This brings
us to the subject of celestial evolution, which
is discussed in another chapter.

CHAPTER V.

THE INNER PLANETS.

MUCH progress has been made during the last
hundred years in our knowledge of the planets.
In fact, the study of Mercury only dates from
the commencement of the nineteenth century.
Our knowledge of the vicinity of the Sun is
very limited, and Mercury is difficult of observa-
tion. So limited, in fact, is our knowledge of
the Sun's surroundings, that it is not yet known
for certain 'whether there is a planet, or planets,
between Mercury and the Sun. Perturbations
in the motion of the perihelion of Mercury's
orbit led Le Verrier in 1859 to the belief that
a planet of about the size of Mercury, or else
a zone of asteroids, existed between Mercury and
the Sun. It was, however, obvious that such a
planet could only be seen when in transit across
the Sun's disc, or during a total eclipse. Mean-
while a French doctor, Lescarbault, informed
Le Verrier that he had seen a round object in

THE INNER PLANETS. 81

transit over the Sun's disc. Le Verrier, certain
that this was the missing planet, named it
"Vulcan," and calculated its orbit, assigning it
a revolution period of twenty days. But it was
never seen again. Transits of "Vulcan" were
fixed for 1877 and 1882, but nothing was seen
on these dates. During the total eclipse of
July 29, 1878, two observers — James Watson
(1838-1880), the well-known astronomer, and
Lewis Swift (born 1820) — believed themselves
to have discovered two separate planets, and
ultimately claimed two planets each, which were
never heard of again. During the total eclipse
of 18 83 an active watch for " suspicious objects "
was kept, but with no result. At the eclipses
of 1900 and 1901 respectively, photographs were
exposed by the American astronomers, W. H.
Pickering and Charles Dillon Perrine (born
1867), but on none of these plates could any
trace of "Vulcan" be found. At the total
eclipse of August 30, 1905, plates were again
exposed, but no announcement has been made
of an intra-Mercurial planet ; and the prevalent
opinion among astronomers is that no planet
comparable with Mercury in size exists between
that planet and the Sun.

The study of the physical appearance of
Mercury was inaugurated by Schroter, who in

F

82 A CENTURY'S PROGRESS IN ASTRONOMY.

1800 noticed that the southern horn of the
crescent presented a blunted appearance, which
he attributed to the existence of a mountain
eleven miles in height. From observations of
this mountain he came to the conclusion that
the planet rotated in 24 hours 4 minutes. This
was afterwards reduced by Friedrich Wilhelm
Beqsel (1784-1846) to 24 hours 53 seconds.

After the time of Schroter there was no
astronomer who paid much attention to either
Mercury or Venus until the arrival on the scene
of the most persistent planetary observer and
one of the foremost astronomers of the nine-
teenth century. Giovanni Virginio Schiaparelli
was born at Savigliano, in Piedmont, in 1835,
and graduated at Turin in 1854. Called to
Milan as assistant in the Brera Observatory in
1860, he became director in 1862, and there
for thirty-eight years he studied astronomy in
all its aspects, making a great name for him-
self in various branches of the science. In
1900 he retired from the post of director, and
pursues his astronomical researches in his
retirement.

In 1882 Schiaparelli took up the study of
Mercury in the clear air of Milan. Instead of
observing the planet through the evening haze,
like Schroter and others, he examined it by day,

THE INNER PLANETS. 83

and was enabled to follow it hourly instead of
looking at it for a short period when near the
horizon. At length, after seven years' observa-
tion, he announced, on December 8, 1889, that
Mercury performs only one rotation during its
revolution round the Sun — in fact, that its day
and year coincide. As a consequence, the planet
keeps the same face towards the Sun, one side
having everlasting day and the other perpetual
night ; but owing to the libratory movement of
Mercury — the result of uniform motion on its
axis and irregular motion in its orbit — the Sun
rises and sets on a small zone of the planet's
surface. Schiaparelli's observations indicated that
Mercury is a much spotted globe, with a moder-
ately dense atmosphere, and he wras enabled to
form a chart of its surface-markings.

Schiaparelli's conclusions remained until 1896
unconfirmed and yet not denied, although most
astronomers were sceptical on the subject. In
1896 the subject was taken up by the American
astronomer, Percival Lowell (born 1855), who, in
the clear air of Arizona, confirmed Schiaparelli's
conclusions, fixing 88 days as the period of
rotation. He remarked, however, that no signs
of an atmosphere or clouds were visible to him.
The surface of Mercury, he says, is colourless, —
" a geography in black and white." The deter-

84 A CENTURY'S PROGRESS IN ASTRONOMY.

mination of the rotation period by Schiaparelli
and Lowell is now generally accepted, and is
confirmed by the theory of tidal friction. It
is only right to add that William Frederick
Denning (born 1848) in 1881 suspected a
rotation period of 25 hours, but this remains
unconfirmed. In April 1871 the spectrum of
Mercury was examined by Hermann Carl
Vogel (born 1842) at Bothkamp. He suspected
traces of an atmosphere similar to ours, but
was not certain. Of more interest are the
photometric observations of Zollner in 1874.
These observations indicated that the surface
of Mercury is rugged and mountainous, and
comparable with the Moon, — a conclusion sup-
ported by Lowell's observations in 1896.

Venus, the nearest planet to the Earth, has
been attentively studied for three centuries, and
still comparatively little is known regarding it.
This is due to its remarkable brilliancy, com-
bined with its proximity to the Sun. The
great problem at the beginning of the nine-
teenth century was the rotation of the planet.
In 1779 the subject was taken up by Schroter
at Lilienthal. Nine years later, from a faint
streak visible on the disc, he concluded that
rotation was performed in 23 hours 28 minutes,
and in 1811 this was reduced by seven minutes ;

THE INNER PLANETS. 85

but as Herschel was unable to observe the mark-
ings seen by Schroter, many astronomers were
inclined to be sceptical regarding the accuracy
of the Lilienthal observer's results. Schroter
also observed the southern horn of Venus when
in the crescent form to be blunted, and he
ascribed this to the existence of a great
mountain, five or six times the elevation of
Chimborazo ; while he observed irregularities
along the terminator, which he considered to be
more strongly marked than those on the Moon.
Schroter's opinion on this point, although re-
jected by Herschel, was confirmed by Madler,
Zenger, Ertborn, Denning, and by the Italian
astronomer Francesco Di Vico (1805-1848),
director of the Observatory of the Collegio
Romano. In 1839 Di Vico attacked the
problem of the rotation, and his results were
confirmatory of those of Schroter. He estimated
that the axis of Venus was inclined at an angle
of 53° to the plane of its orbit. Meanwhile
a series of important observations had been
made on Venus by the Scottish astronomer
and theologian, Thomas Dick (1772-1857), who
suggested daylight observations on Venus to
solve the problem of the rotation.

In 1877 the question was attacked by
Schiaparelli, who commenced a series of ob-

86 A CENTURY'S PROGRESS IN ASTRONOMY.

servations on Venus at Milan in that year.
The results of his studies were summed up in
1890 in five papers contributed to the Milan
Academy. He came to the conclusion that the
markings observed by Schroter, Di Yico, and
others were not really permanent, and concen-
trated his attention on round white spots, which
remained fixed in position. Instead of observing
Venus in the evening, Schiaparelli followed it by
day, watching it continuously on one occasion
for eight hours. But the markings remained
fixed. Schiaparelli accordingly concluded that
the planet's rotation was performed in prob-
ably 225 days, equal to the time of revol-
ution. One face is turned towards the Sun
continually, while the other is perpetually in
darkness.

The announcement was so startling that, as
Miss Clerke says, " a clamour of contradiction
was immediately raised, and a large amount of
evidence on both sides of the question has since
been collected." Perrotin at Nice, Tacchini at
Rome, Cerulli at Teramo, Mascari at Catania
and Mount Etna, and Lowell in Arizona, all
in favourable climates, confirmed Schiaparelli's
results, as also did a second series of observa-
tions by the Milan astronomer himself in 1895.
On the other hand, Neisten, Trouvelot, Camille

THE INNER PLANETS. 87

Flammarion (born 1842), and others, under
less favourable climatic conditions, arrived at
a period of 24 hours. Aristarch Belopolsky
(born 1854), from spectroscopic observations at
Pulkowa, by means of Doppler's principle, found
a period of 12 hours. Lowell, by the same
principle, found, in 1901-03, a period of 225
days, in agreement with Schiaparelli's results.
This is the last word on the subject. Schia-
parelli's rotation period, confirmed by the theory
of tidal friction, is generally accepted.

That Yenus has an atmosphere was one of
the conclusions reached by Schroter in 1792 ;
and in this at least he was correct, as the
atmosphere of Yenus, illuminated by the solar
rays, has been seen extending round the entire
disc of the planet. Spectroscopic observations
by Tacchini, Ricco, and Young, during the
transits of 1874 and 1882, indicated the exist-
ence of water-vapour in the planet's atmosphere.
Yery little has been discovered regarding the
" geography" of Venus. White patches at the
supposed "poles" of the planet were observed
in 1813 by Franz von Gruithuisen, and in 1878
by the French astronomer Trouvelot (1827-1895).
The secondary light of Venus, similar to the
" old Moon in the new Moon's arms," was
repeatedly observed since the time of Schroter

88 A CENTURY'S PROGRESS IN ASTRONOMY.

by Vogel, Lohse, Zenger, and others. Vogel
attributed it to twilight, and Lamp, a German
observer, to electrical processes analogous to our
aurorse. In 1887 a Belgian astronomer, Paul
Stroobant, submitted to a searching examination
all the supposed observations of a satellite of
Venus, and was enabled to explain nearly all
the supposed satellites as small stars which
happened to lie near the planet's path in the
sky at the time of observation.

The study of our own planet can hardly be
said to belong to the realm of astronomy.
Nevertheless, it is through astronomical observa-
tion that the motion of the North Pole has been
discovered. For many years it has been a
problem whether there is a variation of latitude
resulting from the motion of the pole. Euler
had declared, from theoretical investigation, that,
were there such a motion, the period must be
10 months. The question was revived in 1885
by the observations of Seth Carlo Chandler
(born 1846) at Cambridge, Mass., with his
newly -invented instrument, the " almucantar,"
which indicated an appreciable variation of
latitude. This was confirmed by Friedrich
Kiistner (born 1856), now director of the
Observatory at Bonn. The idea now occurred
to Chandler to search through the older records

THE INNER PLANETS. 89

to discover if there was any trace of the varia-
tion of latitude, with the result that he brought
out a period of 14 months instead of 10. This
aroused much interest, and many prominent
astronomers denied Chandler's results, which
were announced in 1891. As a well-known
astronomer has expressed it, " Euler's work
had shown what period the motion must have,
and any appearance of another period must be
due to some error in the observations. Chandler
replied to the effect that he did not care for
Euler's mathematics : the observations plainly
showed 14 months, and if Euler said 10, he
must have made the mistake. I do not ex-
aggerate the situation in the least ; it was a
deadlock : Chandler and observation against
the whole weight of observation and theory."
It was now shown by Newcomb that Euler had
assumed the Earth to be an absolutely rigid
body, while modern investigations show that it
is not so. Chandler's discovery is now accepted,
and proves that the North Pole is not fixed
in position, but has a small periodic motion,
though never twelve yards from its mean posi-
tion. That the small resulting variation in the
position of the stars has been noticed at all
is a striking illustration of the accuracy of
astronomical observation.

90 A CENTURY'S PROGRESS IN ASTRONOMY.

Of all the planets Mars has been most studied
during the nineteenth century. Many illustrious
astronomers have devoted years to the study of
the red planet, with the result that more is
known of the surface of Mars than of any other
celestial body, with the exception of the Moon.
After the time of Herschel, the leading students
of Mars were Beer and Madler, who carefully
studied the planet from 1828 to 1839. They
identified at each opposition the same dark spots,
frequently obscured by mists, and they also made
the most accurate determination of the rotation
period, which they fixed at 24 hours 37 minutes 23
seconds. This estimate was confirmed in 1862 by
Friedrich Kaiser (1808-1872) of Leyden, in 1869
by Richard Anthony Proctor (1837-1888), and
in 1892 by Henricius Gerardus van de Sande
Bakhuyzen (born 1838), director of the Leyden
Observatory. In 1862 Lockyer identified the
various markings seen by Beer and Madler in
1830. The other great names in Martian study
prior to 1877 are Angelo Secchi and William
Rutter Dawes (1799-1868), who studied Mars
from 1852 to 1865 and secured a very valu-
able series of drawings. These drawings were
used by Proctor for the construction of the
first reliable map of Mars, which was published
in 1870 in his work, 'Other Worlds than Ours/

THE INNER PLANETS. 91

Proctor gave names to the various Martian
features, the reddish-ochre portions of the disc
being named continents and the bluish-green
portions seas ; and Proctor's views on Mars
found favour for many years. In 1877, how-
ever, Schiaparelli opened a new era in the study
of Mars. In September of that year, during the
very favourable opposition of the planet, Schia-
parelli, while executing a trigonometrical survey
of the disc, discovered that the continents were
cut up by numerous long dark streaks, which he
called canali. In 1879, to his surprise, he found
that some of the canals had become double ;
and he confirmed this in 1881 and at subsequent
oppositions. Meanwhile, as Schiaparelli was the
only observer who had hitherto seen the canals,
there was much scepticism as to their reality.
In 1886, however, they were seen at the Nice
Observatory by Henri Perrotin (1845-1904),
who also observed their duplication. Since
1886 they have been observed by many astron-
omers, including Camille Flammarion in France,
William Frederick Denning (born 1848) in
England, Vincenzo Cerulli (born 1859) in Italy,
Percival Lowell and W. H. Pickering in the
United States. In 1892 W. H. Pickering suc-
cessfully observed the canals, and discovered
at the junctions of two or more canals round

92 A CENTURY'S PROGRESS IN ASTRONOMY.

black spots, to which he gave the name of
" lakes," in keeping with the view that the dark
regions of the planet were seas.

In 1894 Percival Lowell erected at Flagstaff,
Arizona, an observatory for the specific purpose
of observing Mars and its canals in good and
steady air. He was assisted by W. H. Pickering
and by Andrew Ellicott Douglass (born 1867).
During a year's study Douglass measured the
Martian atmosphere and discovered canals cross-
ing the dark regions of the planet, finally dis-
proving the idea of their aqueous character.
Lowell recognised all Schiaparelli's canals, and
discovered many more. He also attentively
studied the south polar cap of Mars, which dis-
appeared entirely on October 12, 1894. Lowell
noticed, also, that as the cap melted the canals
became darker, as if water was being conveyed
down; and accordingly he adopted the view
put forward by Schiaparelli, that the canals are
waterways lined on either side by banks of
vegetation. His observations were published in
the end of 1895 in his work 'Mars/ He is of
opinion that the reddish-ochre regions or "con-
tinents" are deserts, and the greenish areas
marshy tracts of vegetation. The lakes are
named by him "oases," and, as Miss Clerke
observes, he " does not shrink from the full

THE INNER PLANETS. 93

implication of the term." He regards the canals
as strips of vegetation fertilised by a small
canal, much too small to be seen, an idea which
originated with W. H. Pickering. The canals
are believed by Lowell to be waterways down
which the water from the melting polar cap is
conveyed to the various oases. He considers, in
fact, that the canals are constructed by intelli-
gent beings with the express purpose of fertil-
ising the oases, regarded by him as centres of
population. He remarks that water is scarce on
the planet, owing to its small size, and as a con-
sequence the inhabitants are forced to utilise every
drop. The canal system is the result.

Lowell's theory has not been cordially received
— although it is now gradually gaining popu-
larity,— and several other hypotheses have been
propounded to explain the canals. Proctor, who
died some years before Lowell's theory was given
to the world, regarded them as rivers, but this
view may now be looked upon as abandoned.
It was suggested that the canals might be cracks
in the surface of Mars or meteors ploughing
tracks above it : and Professor John Martin
Schaeberle (born 1853) of the Lick Observatory
put forward the view that the canals were chains
of mountains running over the light and dark
regions. None of these theories, however, gained

94 A CENTURY'S PROGRESS IN ASTRONOMY.

popularity, and had to give way to a more
popular theory, the " illusion " hypothesis, put
forward by the Italian astronomer Cerulli, and
supported by Newcomb and Maunder. On the
basis of the illusion theory, Newcomb explains
that the " canaliform " appearance "is not to be
regarded as a pure illusion on the one hand or
an exact representation of objects on the other.
It grows out of the spontaneous action of the
eye in shaping slight and irregular combinations
of light and shade, too minute to be separately
made out into regular forms." Experiments
were made by Maunder in 1902, and the results
pointed to the truth of the theory that the
canals were really illusions. But the studies of
Lowell at the oppositions of 1903 and 1905 have
seriously weakened the hypothesis of Cerulli and
Maunder, and strongly confirm the theory of the
artificial origin of the canals. In 1903 Lowell was
enabled, from a study of the development of the
canals, to show the probability of their artificial
nature, and his study of the double canals showed
a distinct plan in their distribution. Finally,
on May 11, 1905, several photographs of Mars
were secured at the Lowell Observatory, on
which the canals appeared, not as dots of light
and shade, as on the illusion theory, but as
straight dark lines. This goes far to prove the

THE INNER PLANETS. 95

reality of the canals, — in spite of the ridicule
cast on them and their observers, — and conse-
quently the truth of the theory of intelligent life
in Mars.

Meanwhile the old-fashioned Martian obser-
vations have been continued in less favourable
climates than Arizona and Italy by various
astronomers, among them the famous Camille
Flammarion, the American astronomers James
Edward Keeler (1857-1900), Edward Emerson
Barnard (born 1857), the English astronomer
W. F. Denning, and others. These conscientious
and painstaking observers have done much for
Martian study in increasing the number of
accurate delineations of the Martian surface.

The spectrum of Mars was first examined by
Huggins in 1867. He found distinct traces of
water-vapour, and this was confirmed by Vogel
in 1872, and by Maunder some years later. In
1894, however, William Wallace Campbell (born
1862), the American astronomer, observing from
the Lick Observatory, California, was unable to
detect the slightest difference between the spectra
of Mars and the Moon, indicating that Mars had
no appreciable atmosphere ; and from this he de-
duced that the Martian polar caps could not be
composed of snow and ice, but of frozen carbonic
acid gas. In 1895, however, Vogel confirmed his

96 A CENTURY'S PROGRESS IN ASTRONOMY.

previous observations, and reaffirmed the presence
of water- vapour in the Martian atmosphere.

During the opposition of 1830, Madler under-
took an extensive search for a Martian satellite,
but was unsuccessful. In 1862 the search was
resumed by Heinrich Louis U Arrest (1822-1875),
the famous German observer, who was also un-
successful. Accordingly the red planet was re-
ferred to by Tennyson as the "moonless Mars."
In 1877 the search was taken up by Asaph
Hall, the self-made American astronomer, born
at Goshen, Connecticut, in 1829, and employed
from 1862 to 1891 at the Naval Observatory,
Washington. During the famous opposition of
August 1877, favoured by the great 26 -inch
refractor, he succeeded in discovering two very
small satellites of Mars, to which he gave the
names of Phobos and Deimos. He determined
the time of revolution of Phobos at 7 hours
39 minutes, and that of Deimos at 30 hours
17 minutes, — Phobos revolving round Mars more
than three times for one rotation of the planet
on its axis. These two satellites are very small,
not more than thirty miles in diameter. After
Hall's successful search, photographs were ex-
posed at the Paris Observatory for other Martian
satellites, but none was discovered. No further
moons have been found belonging to the red

THE INNER PLANETS. 97

planet, nor is it likely that any further satellites
of Mars are in existence.

The discovery of a zone of small planets in
the space between Mars and Jupiter belongs
completely to the nineteenth century, although
the existence of a planet in the vacant space
was suspected three centuries ago. In 1772 the
subject was taken up by Johann Elert Bode
(1747-1826), afterwards director of the Berlin
Observatory, who investigated a curious numer-
ical relationship, since known as Bode's Law, con-
necting the distances of the planets. If four is
added to each of the numbers — 0, 3, 6, 12, 24,
48, 96, and 192, the resulting series represents
pretty accurately the distances of the planets
from, the Sun, thus — 4 (Mercury), 7 (Venus),
10 (The Earth), 16 (Mars), 28, 52, (Jupiter),
and 100 (Saturn). After the discovery of
Uranus, in 1781, it was found that it filled up
the number 196. Bode, however, saw that
the number 28, between Mars and Jupiter, was
vacant, and predicted the discovery of the planet.
Aided by Franz Xavier von Zach (1754-1832),
he called a congress of astronomers, which as-
sembled in 1800 at Schroter's observatory at
Lilienthal, when, for the purpose of searching
for the missing planet, the zodiac was divided
into twenty-four zones, each of which was given

G

98 A CENTURY'S PROGRESS IN ASTRONOMY.

to a separate astronomer. One of them was
reserved for Giuseppe Piazzi (1746-1826), director
of the Observatory of Palermo.

Born in 1746 at Ponte, in Lombardy, Giuseppe
Piazzi, after entering the Theatine Order of
monks, became in 1780 Professor of Mathematics
at Palermo, where an observatory was erected
in 1791 ; and at that observatory Piazzi worked
till his death in 1826. In 1792 he commenced
a great star -catalogue, and while making his
nightly observations he discovered, on January 1,
1801 — the first night of the nineteenth century,
— what he took to be a tailless comet, but which
proved to be a small planet revolving round
the sun in the vacant space. The discovery
was hailed by Bode and Von Zach with much
enthusiasm, and Piazzi named the planet Ceres.
The little planet was, however, soon lost in the
rays of the sun before sufficient observations
had been made ; but the great mathematician,
Friedrich Gauss (1777-1855), came to the rescue,
and pointed out the spot where the planet was
to be rediscovered. In that spot it was found
on December 31, 1801, by Yon Zach at Gotha,
and on the following evening by Heinrich Gibers
(1758-1840) at Bremen.

On March 28, 1802, while observing Ceres

THE INNER PLANETS. 99

from his house at Bremen, Olbers was struck
by the presence of a strange object near the
path of the planet. At first he supposed it to
be a variable star at maximum brilliance, but
a few hours showed him that it was in motion,
and was therefore another planet. He named
it Pallas, and propounded the theory that the
two "Asteroids" — so named by Herschel — were
fragments of a trans - Martian planet, which,
through some accident, had been shattered to
pieces in the remote past. Olbers urged the
necessity of searching for more small planets.
His advice was taken. In 1804 Karl Ludwig
Harding (1765-1834), Schroter's assistant, dis-
covered Juno, and Olbers himself detected Vesta,
March 29, 1807.

After 1816 the search was relinquished, as
no more planets were discovered. In 1830,
however, a German amateur, Karl Ludwig
Hencke (1793-1866), ex-postmaster of Driessen,
commenced a search for new planets, which was
rewarded, after fifteen years, by the discovery
of Astrsea, December 8, 1845. On July 1, 1847,
he made another discovery, that of Hebe. A few
weeks later, John Russell Hind (1823-1895), the
English astronomer, discovered Iris. Since 1847
not a year has passed without one or more planets

100 A CENTURY'S PROGRESS IN ASTRONOMY.

being found, sometimes as many as twenty being
discovered in a single year. Some astronomers
have made the search for asteroids their chief
business. The principal asteroid discoverers have
been Christian H. F. Peters (1813-1890), Henri
Perrotin, Paul Henry (1848-1905), Prosper
Henry (1849-1903), James Watson, Robert
Luther (1822-1900), Johann Palisa (born 1848),
and Max Wolf (born 1863).

In 1891 a new impulse was given to asteroid
study by the application of photography by Max
Wolf to the discovery of the minor planets. It
occurred to Wolf that the asteroid would be
represented on the plate by a trail, caused by
its motion during the time of exposure ; and
assisted by Arnold Schwussmann (born 1870),
Luigi Camera (born 1875), and others, Wolf has
discovered over a hundred asteroids, and he
has the whole field of asteroid hunting to him-
self. Few minor planets are now discovered by
the older method. In 1901 Wolf invented his
new instrument of research, the stereo -com-
parator, which, on the principle of the old-
fashioned stereoscope, represents the planetary
bodies as suspended in space far in front of the
stars. In this way this ingenious astronomer
has been enabled to discover asteroids at the

THE INNER PLANETS. 101

first glance : year by year fresh discoveries are
announced from the Heidelberg Observatory,
until more than five hundred asteroids are now
known.

Waning interest in the ever- increasing family
of asteroids was revived in 1898 by the dis-
covery by Karl Gustav Witt (born 1866) of a
small planet, to which he gave the name of Eros,
which comes nearer to the Earth than Mars, and
which is of great assistance to astronomers in the
determination of the solar parallax. For some
time prior to 1898 astronomers had considered
it a waste of time to search for new asteroids ;
but this idea is not now so popular, in view
of the benefit conferred on astronomy by the
discovery of Eros.

Of the physical nature of the asteroids astron-
omers know nothing. Only the four largest
have been measured. For many years it was
supposed that Vesta, the brightest of the aster-
oids, was also the largest. The measures of
Barnard with the great Lick refractor in 1895,
however, showed that Ceres is the largest, with
a diameter of 477 miles. Pallas comes next,
with a diameter of 304 miles ; while the dia-
meters of Vesta and Juno are respectively 239
and 120 miles. Barnard saw no traces of atmo-

102 A CENTURY'S PROGRESS IN ASTRONOMY.

sphere round any of the asteroids. It should
be stated that in 1872 Vogel thought he could
detect an " air-line " in the spectrum of Vesta :
he admitted that the observation required con-
firmation, but it has not been corroborated either
by himself or any other observer.

CHAPTER VI.

THE OUTER PLANETS.

JUPITER, the greatest planet of the Solar System,
has perhaps been more persistently studied by
astronomers than any other. In the early
nineteenth century the prevalent idea was that
Jupiter was a world similar to the Earth, only
much larger, — a view held by Herschel and other
famous astronomers, and put forward by Brewster
in ' More Worlds than One/ This view prevailed
for many years, although Buffon in 1778, and
Kant in 1785, had stated their belief in the idea
that Jupiter was still in a state of great heat —
in fact, that the great planet was a semi-sun.
This idea, however, was long in being adopted
by astronomers, and very little attention was
paid to Nasmyth's expression of the same opinion
in 1853. The older view still held the field —
namely, that the belts of Jupiter represented
trade -winds, and that a world similar to the
terrestrial lay below the Jovian clouds. In 1860

104 A CENTURY'S PROGRESS IN ASTRONOMY.

George Philip Bond (1826-1865), director of the
Harvard Observatory, found from experiments
that Jupiter seemed to give out more light than
it received, but he did not dare to suggest that
Jupiter was self-luminous, considering that the
inherent light might result from Jovian auroras.

In 1865 Zollner showed that the rapid motions
of the cloud-belts on both Jupiter and Saturn
indicated a high internal temperature. At the
distance of Jupiter sun-heat is only one twenty-
seventh as great as on the Earth, and would be
quite incapable of forming clouds many times
denser than those on the Earth. In 1871
Zollner drew attention to the equatorial accel-
eration of Jupiter, analogous to the same phe-
nomenon on the Sun. In 1870 these opinions of
Zollner' s were adopted and supported by Proctor
in his ' Other Worlds than Ours.' In his subse-
quent volumes Proctor did much to popularise
the idea, which is now accepted all over the
astronomical world.

During the century many valuable observations
on Jupiter were made by numerous observers,
among them Airy, Madler, Webb, Schmidt, and
others. Much time was devoted to the accurate
determination of the rotation period, which was
fixed at 9 hours 55 minutes 3 6 '5 6 seconds by
Denning in observations from 1880 to 1903. No

THE OUTER PLANETS. 105

really important discovery was made till 1878,
when Niesten at Brussels discovered the "great
red spot," a ruddy object 25,000 miles long by
7000 broad, attached to a white zone beneath
the southern equatorial belt. This remarkable
object has been observed ever since. In 1879 its
colour was brick-red and very conspicuous, but
it soon began to fade, and Bicco's observation
at Palermo in 1883 was thought to be the last.
After some months, however, it brightened up,
and, notwithstanding changes of form and colour,
it is still visible, a permanent feature of the
Jovian disc. In 1879 a group of " faculae,"
similar to those on the Sun, was observed at
Moscow by Theodor Alexandrovitch JBredikhine
(1831-1904), and at Potsdam by Wilhelm Oswald
Lohse (born 1845). It was soon observed that
the rotation period, as determined from the great
red spot, was not constant, but continually in-
creasing. A white spot in the vicinity completed
its rotation in 5J minutes less, indicating the
differences of rotation on Jupiter.

The great red spot has been observed since
its discovery by Denning at Bristol and George
Hough (born 1836) at Chicago. Twenty-eight
years of observation have not solved the mystery
of its nature. The researches made on it, in the
words of Miss Clerke, " afforded grounds only

106 A CENTURY'S PROGRESS IN ASTRONOMY.

for negative conclusions as to its nature. It
certainly did not represent the outpourings of a
Jovian volcano ; it was in no sense attached to
the Jovian soil — if the phrase have any applica-
tion to the planet ; it was not a mere disclosure
of a glowing mass elsewhere seethed over by
rolling vapours."

In 1870 Arthur Cowper Ranyard (1845-1894),
the well-known English astronomer, began to
collect records of unusual phenomena on the
Jovian disc to see if any period regulated their
appearance. He came to the conclusion that,
on the whole, there was harmony between the
markings on Jupiter and the eleven-year period
on the Sun. The theory of inherent light in
Jupiter, however, has not been confirmed. The
great planet was examined spectroscopically by
Huggins from 1862 to 1864, and by Vogel from
1871 to 1873. The spectrum showed, in addition
to the lines of reflected sunlight, some lines
indicating aqueous vapour, and others which
have not been identified with any terrestrial sub-
stance. A photographic study of the spectrum
of Jupiter was made at the Lowell Observatory
by Slipher in 1904, probably the most exhaustive
investigation on the subject. The spectroscope
has, however, given little support to the theory
of inherent light, and " we are driven to con-

THE OUTER PLANETS. 107

elude that native emissions from Jupiter's visible
surface are local and fitful, not permanent and
general."

Herschel's idea, that the rotations of the four
satellites of Jupiter were coincident with their
revolutions, has on the whole been confirmed
by recent researches, although in the case of
the two near satellites (lo and Europa) W. H.
Pickering's observations in 1893 indicated shorter
rotation periods. There is much to learn re-
garding the geography of the satellites, although
in 1891 Schaeberle and Campbell at the Lick
Observatory observed belts on the surface of
Ganymede, the third satellite analogous to those
on Jupiter. Surface-markings on the satellites
have also been seen by Barnard at the Lick
Observatory, and by Douglass at Flagstaff.

Since the time of Galileo no addition had been
made to the system of satellites revolving round
Jupiter. Profound surprise was created, there-
fore, by the announcement of the discovery of
a fifth satellite by Barnard at the Lick Observ-
atory, on September 9, 1892. The satellite,
one of the faintest of telescopic objects, was
discovered with the great 3 6 -inch telescope, and
its existence was soon confirmed by Andrew
Anslie Common (1841-1903), with his great 5-
foot reflector at Ealing, near London. The new

108 A CENTURY'S PROGRESS IN ASTRONOMY.

satellite was found by Barnard to revolve round
Jupiter in 11 hours 57 minutes at a mean dis-
tance of 112,000 miles.

Although the existence of other satellites of
Jupiter was predicted by Sir Robert Stawell Ball
(born 1840) soon after the discovery of the fifth,
much surprise was created by the announcement,
in January 1905, that a sixth satellite had been
discovered by Perrine, who, in the following
month, announced the discovery of a seventh.
These discoveries were made by photography,
the objects being very faint. The periods of
revolution were found to be 242 days and 200
days for the sixth and seventh satellites re-
spectively, the mean distances being 6,968,000
and 6,136,000 miles. It is possible that they
may belong to a zone of asteroidal satellites.
In fact, the fifth moon may belong to a similar
zone, so that Jupiter may have two asteroidal
zones ; but this is anticipating future discovery.

A particular charm has always attached itself
to the study of Saturn, the ringed planet. The
magnificent system of rings has for two and a
half centuries been the object of wonder and
admiration in the Solar System, and accordingly
they have been exhaustively studied by many
eminent observers. While observing the two
bright rings of Saturn on June 10, 1838, Galle

THE OUTER PLANETS. 109

noticed what Miss Clerke calls " a veil - like
extension of the lucid ring across half the dark
space separating it from the planet." No atten-
tion, however, was paid to Galle's observation.
On November 15, 1850, William Cranch Bond
(1789-1859), of the Harvard Observatory in
Massachusetts, discovered the same phenomenon
under its true form — that of a dusky ring in-
terior to the more brilliant one. A fortnight
later, before the news of Bond's observation,
Dawes made the same discovery independently
at Wateringbury in England. This ring is
known as the dusky or "crape" ring.

The discovery of the dusky ring brought to
the front the problem of the composition of the
ring- system. Laplace and Herschel considered
the rings to be solid, but this was denied in 1848
by Edouard Roche (1820-1880), who believed
them to consist of small particles, and in 1851
by G. P. Bond, who asserted that the variations
in the appearance of the system were sufficient
to negative the idea of their solidity ; but he
suggested that the rings were fluid. In 1857
the question was taken up by the Scottish
physicist, James Cleric - Maxwell (1831-1879),
who proved by mathematical calculation that
the rings could be neither solid nor fluid, but
were due to an aggregation of small particles,

110 A CENTURY'S PROGRESS IN ASTRONOMY.

so closely crowded together as to present the ap-
pearance of a continuous whole. Clerk-Maxwell's
explanation — which had been suggested by the
younger Cassini in 1715, and by Thomas Wright
in 1750 — was at once adopted, and has since
been proved by observation. In 1888 Hugo
Seeliger (born 1849), director of the Munich
Observatory, showed from photometric observa-
tions the correctness of the satellite -theory;
while Barnard in 1889 witnessed an eclipse of
the satellite Japetus by the dusky ring. The
satellite did not disappear, but was seen with
perfect distinctness. The final demonstration of
the meteoric nature of the rings was made by
Keeler at the Alleghany Observatory in 1895,
with the aid of the spectroscope. By means
of Doppler's principle, he found that the inner
edge of the ring revolved in a much shorter
time than the outer, proving conclusively that
they could not be solid. This was confirmed by
the observations of Campbell at Mount Hamilton,
Henri Deslandres at Meudon, and Belopolsky
at Pulkowa.

In 1851 a startling theory regarding Saturn's
rings was put forward by the famous Otto
Wilhelm von Struve (1819-1905). Comparing his
measurements on the rings made at Pulkowa in
1850 and 1851 with those of other astronomers

THE OUTER PLANETS. Ill

for the past two hundred years, he reached the
conclusion that the inner diameter of the ring
was decreasing at the rate of sixty miles a-year,
and that the bodies composing the rings were
being drawn closer to the planet. Accordingly,
Struve calculated that only three centuries would
be required to bring about the precipitation of
the ring-system on to the globe of Saturn. In
1881 and 1882 Struve, expecting a further
decrease, made another series of measures, but
these did not confirm his theory, which was
accordingly abandoned.

The study of the globe of Saturn has made
less progress than that of the rings. The surface
of the planet had been known since before the
time of Herschel to be covered with belts, but
as spots seldom appear on Saturn, only one deter-
mination of the rotation period had been made,
that by Herschel. Much interest was aroused,
therefore, by the discovery, by Hall, at Washing-
ton, on December 7, 1876, of a bright equatorial
spot. Hall studied this spot during sixty rota-
tions of the planet, determining the period as
10 hours 14 minutes 24 seconds. This was con-
firmed by Denning in 1891, and by Stanley
Williams, an English observer, in the same year.
On June 16, 1903, Barnard, at the Yerkes
Observatory, discovered a bright spot, from

112 A CENTURY'S PROGRESS IN ASTRONOMY.

which he deduced a rotation period of 10 hours
39 minutes, — a period considerably longer than
that found by Hall. In the same year various
spots on Saturn were observed by Denning, who
found a period of 10 hours 37 minutes 56*4
seconds, and at Barcelona by Jose Comas Sola,
now director of the Observatory there, who
may be considered Spain's leading astronomer.
The result of these observations has been to
show that the spots on Saturn have probably
a proper motion of their own, apart from the
rotation of the planet. As to the spectrum
of Saturn, little has been learned. It closely
resembles that of Jupiter. In 1867 Janssen,
observing from the summit of Mount Etna,
found traces of aqueous vapour in the planet's
atmosphere.

In the chapters on Herschel we have seen that
he discovered the sixth and seventh satellites
of Saturn. The next discovery was made on
September 19, 1848, by W. C. Bond, at
Harvard, Massachusetts, and independently by
William Lassell (1799-1880), at Starfield, near
Liverpool. The new satellite received the name
of Hyperion, and was found to be situated at
a distance of about 946,000 miles from Saturn.
Its small size led Sir John Herschel to the
idea that it might be an asteroidal satellite.

THE OUTER PLANETS. 113

Fifty years elapsed before another satellite of
Saturn was discovered. In 1888 W. H. Picker-
ing commenced a photographic search for new
satellites of the planet. At last, on developing
some photographs of Saturn, taken on August 16,
17, and 18, 1898, he found traces of a new
satellite which he named " Phoebe." But, as the
satellite was not seen or photographed again for
some years, many astronomers were sceptical as
to its existence. However, photographs taken
in 1900, 1901, and 1902 revealed the satellite,
which was again photographed in 1904, and seen
visually by Barnard in the same year with the
40-inch Yerkes telescope. At that time the dis-
coverer brought out the amazing fact that the
motion of the satellite is retrograde — a fact which
he attempts to explain by a new theory of the
former rotation of Saturn. He likewise demon-
strated that its distance from Saturn varied
from 6,120,000 to 9,740,000 miles. Early in 1905
Pickering announced the discovery of a tenth
satellite of Saturn, which received the name of
Themis, with a period and mean distance nearly
similar to Hyperion, so that Sir John Herschers
idea of Hyperion being an asteroidal satellite is
being confirmed after a lapse of half a century.

If little is known of the globe of Saturn, still
less is known regarding Uranus. Dusky bands

114 A CENTURY'S PROGRESS IN ASTRONOMY.

resembling those of Jupiter were observed by
Young at Princeton in 1883. In the following
year Paul and Prosper Henry discerned at Paris
two grey parallel lines on the disc of the planet.
This was confirmed by the observations of Per-
rotin at Nice, which also indicated rotation in a
period of ten hours. In 1890 Perrotin again took
up the study and re-observed the dark bands.
On the other hand, no definite results regarding
the planet were obtained by the Lick observers
in 1889 and 1890. Measurements of the planet
by Young, Schiaparelli, Perrotin, and others
indicate a considerable polar compression. The
spectrum of the planet has been studied by
Secchi, Huggins, Vogel, Keeler, Slipher, and
others. The spectrum shows six bands of
original absorption, a line of hydrogen, which,
says Miss Clerke, " implies accordingly the
presence of free hydrogen in the Uranian atmo-
sphere, where a temperature must thus prevail
sufficiently high to reduce water to its con-
stituent elements." From a photographic study
of the spectrum at the Lowell Observatory in
1904, Slipher observed a line corresponding to
that of helium, indicating the presence of that
element in the planet's atmosphere.

Herschel left our knowledge of the Uranian
satellites in a very uncertain state. The two

THE OUTER PLANETS. 115

outer satellites, Titania and Oberon, were re-
discovered in 1828 by his son, but the other
four, which he was believed to have discovered,
were never seen again. In 1847 two inner
satellites, Ariel and Umbriel, were discovered
by Lassell and Otto Struve respectively, their
existence being finally confirmed by LasselFs
observations in 1851.

After the discovery of Uranus by Herschel,
mathematical astronomers determined its orbit
and calculated its position in the future. Alexis
Bouvard, the calculating partner of Laplace,
published tables of the planet's motions, founded
on observations made by various astronomers
who had considered it a star before its discovery
by Herschel ; but as the planet was not in the
exact position which Bouvard predicted, he
rejected the earlier observations altogether. For
a few years the planet conformed to the French-
man's predictions, but shortly afterwards it was
again observed to move in an irregular manner,
and the discrepancy between observation and
the calculations of mathematicians became intol-
erable. Did the law of gravitation not hold
good for the frontiers of the Solar System ?
Gradually astronomers arrived at the conclusion
that Uranus was being attracted off its course
by the influence of an unseen body, an exterior

116 A CENTURY'S PROGRESS IN ASTRONOMY.

planet. Bouvard himself was one of the first to
make the suggestion, but died before the planet
was discovered. An English amateur, the Rev.
T. J. Hussey, resolved to make, in 1834, a deter-
mination of the place of the unseen body, but
found his powers inadequate ; and in 1840 Bessel
laid his plans for an investigation of the problem,
but failing health prevented him carrying out
his design.

In 1841 a student at the University of Cam-
bridge resolved to grapple with the problem.
John Couch Adams, born at Lidcot in Cornwall
in 1819, entered in 1839 the University of Cam-
bridge, where he graduated in 1843. From 1858
Professor of Astronomy at Cambridge, and from
1861 director of the Observatory, he died on
January 21, 1892, after a life spent in devo-
tion to mathematical astronomy. In 1843, on
taking his degree, he commenced the investi-
gation of the orbit of Uranus. For two years
he worked at the difficult question, and by
September 1845 came to the conclusion that a
planet revolving at a certain distance beyond
Uranus would produce the observed irregulari-
ties. He handed to James Challis (1803-1882),
the director of the Cambridge Observatory, a
paper containing the elements of what was
named by Adams " the new planet." On

THE OUTER PLANETS. 11 7

October 21 of the same year he visited Green-
wich Observatory, and left a paper containing
the elements of the planet, and approximately
fixing its position in the heavens. But the
Astronomer-Royal of England, Sir George Biddell
Airy (1801-1892), had little faith in the calcula-
tions of the young mathematician. He always
considered the correctness of a distant mathe-
matical result to be a subject rather of moral
than of mathematical evidence : in fact, regard-
ing Uranus, the Astronomer-Royal almost called
in question the correctness of the law of gravita-
tion. Besides, the novelty of the investigations
aroused scepticism, and the fact that Adams
was a young man, and inexperienced, went
against Airy's acceptance of the theory. How-
ever, he wrote to Adams questioning him on
the soundness of his idea. Adams thought the
matter trivial, and did not reply. Airy, there-
fore, took no interest in the investigations, and
no steps were taken to search for the unseen
planet. Meanwhile the Rev. W. R. Dawes
happened to see Adams' papers lying at Green-
wich, and wrote to his friend, the well-known
astronomer Lassell, who was in possession of a
very fine reflector, erected at his residence near
Liverpool, asking him to search for the planet.
But Lassell was suffering from a sprained ankle,

118 A CENTURY'S PROGRESS IN ASTRONOMY.

and Dawes' letter was accidentally destroyed by
a housemaid. So Adams' theory remained in
obscurity.

The question now came under the notice of
Francois Jean Dominique Arago (1786-1853),
the director of the Paris Observatory. He
recognised in a young friend of his a rising
genius, who was competent to solve the problem.
Urban Jean Joseph Le Verrier, born at Saint Lo,
in Normandy, in 1811, became in 1837 astro-
nomical teacher in the Ecole Polytechnique, and
in 1853 director of the Paris Observatory. In
consequence of differences with his staff he was
obliged, in 1870, to resign from this position, but
two years later was restored to the post, which
he held till his death on September 23, 1877.

In 1845, ignorant of the fact that Adams had
already solved the problem, Le Verrier began
his investigations of the irregular motions of
Uranus. In a memoir communicated to the
Academy of Sciences in November of that year,
he demonstrated that no known causes could
produce these disturbances. In a second memoir,
dated June 1, 1846, he announced that an ex-
terior planet alone could produce these effects.
But Le Verrier had now before him the difficult
task of assigning an approximate position to the
unseen body, so that it might be telescopically

THE OUTER PLANETS. 119

discovered. After much calculation Le Verrier,
in his third memoir (August 31, 1846), assigned
to the planet a position in the constellation
Aquarius.

Meanwhile one of Le Terrier's papers happened
to reach Airy. Seeing its resemblance to Adams'
papers, which had been lying on his desk for
months, his scepticism vanished, and he sug-
gested to Challis that the planet should be
searched for with the Cambridge equatorial. In
July 1846 the search was commenced. The
planet was actually observed on August 4 and
12, but, owing to the absence of star maps, it
was not recognised. " After four days of observ-
ing," he wrote to Airy, " the planet was in my
grasp if I had only examined or mapped the
observations."

Le Verrier wrote to Encke, the illustrious
director of the Berlin Observatory, desiring him
to make a telescopic search for a planetary object
situated in the constellation Aquarius, as bright
as a star of the eighth magnitude and possessed
of a visible disc. " Look where I tell you,"
wrote the French astronomer, " and you will see
an object such as I describe." Encke ordered
his two assistants, Galle and D' Arrest, to make
a search on the night of September 23, 1846.
In a few hours Galle observed an object not

120 A CENTURY'S PROGRESS IN ASTRONOMY.

marked in the star-maps of the Berlin Observa-
tory, which had been recently published. The
following night sufficed to show that the object
was in motion, and was therefore a new planet.
On September 29 Challis found the planet at
Cambridge, but he was too late, as the priority
of the discovery was now lost to Adams. The
planet received the name of "Neptune."

For some time, indeed, it appeared as if the
French astronomer alone was to receive the
honour of the discovery. But on October 3,
1846, a letter from Sir John Herschel appeared
in the ' Athenaeum ' in which he referred to the
discovery made by Adams. The French scientists
were extremely jealous. Indeed, Arago actually
declared that, when Neptune was under dis-
cussion, the entire honour should go to Le
Verrier, and the name of Adams should not even
be mentioned, — Arago's line of reasoning being
that it was not the man who first made a dis-
covery who should receive the credit, but he
who first made it public. However, the credit
of the discovery is now given equally to Adams
and Le Verrier, both of whom are regarded as
among the greatest of astronomers.

Only a fortnight after the discovery of Nep-
tune, the astronomer Lassell observed a satellite
to the distant planet on October 10, 1846. This

THE OUTER PLANETS. 121

discovery was confirmed in July 1847 by the
discoverer himself, and shortly afterwards by
Bond and Otto Struve. Regarding the globe
of Neptune, we know practically nothing. No
markings of any kind have been observed on its
surface. However, in 1883 and 1884, Maxwell
Hall, an astronomer in Jamaica, noticed certain
variations of brilliance which suggested a rota-
tion-period of eight hours, but this was not
confirmed by any other astronomer. The spect-
rum of Neptune has been investigated by various
observers, who have found it to be similar to
that of Uranus.

The existence of a trans -Neptunian planet
has been suspected by many astronomers. In
November 1879 the first idea of its existence
was thrown out by Flammarion in his ' Popular
Astronomy.' Flammarion noticed that all the
periodical comets in the Solar System have
their aphelion near the orbit of a planet. Thus
Jupiter owns about eighteen comets ; Saturn
owns one, and probably two ; Uranus two or
three ; and Neptune six. The third comet of
1862, however, along with the August meteors,
goes farther out than the orbit of Neptune.
Accordingly, Flammarion suggested the existence
of a great planet, assigning it a period of 330
years and a distance of 4000 millions of miles.

122 A CENTURY'S PROGRESS IN ASTRONOMY.

Two independent investigators, David Peck
Todd (born 1855) in America and George
Forbes in Scotland, have since undertaken to
find the planet. Todd, utilising the "residual
perturbations" of Uranus, assigned a period of
375 years for his planet. Forbes, on the other
hand, working from the comet theory, stated
his belief in the existence of two planets with
periods of 1000 and 5000 years respectively.
In October 1901 he computed the position of
the new planet on the celestial sphere, fixing
its position in the constellation Libra, and
computing its size to be greater than Jupiter.
A search was made by means of photography,
in 1902, but without success. Nevertheless,
astronomers are pretty confident of the exist-
ence of one or more trans -Neptunian planets.
Lowell is very definite on this subject when
he says in regard to meteor groups, " The
Perseids and the Lyrids go out to meet the
unknown planet, which circles at a distance of
about forty-five astronomical units from the Sun.
It may seein strange to speak thus confidently
of what no mortal eye has seen, but the finger
of the sign-board of phenomena points so clearly
as to justify the definite article. The eye of
analysis has already suspected the invisible."

CHAPTER VII.

COMETS.

AT the time of Herschel the ancient super-
stitions in regard to comets had to a great
extent vanished, thanks mainly to the return
of Halley's comet in 1758. Yet, although
comets had ceased to be objects of terror, no
explanation or rational theory of their nature
was put forward until the appearance of the
great comet of 1811. This comet was visible
from March 26, 1811, to August 17, 1812, a
period of 510 days. It was one of the most
magnificent comets ever seen, its tail being
100 millions of miles in length and its head
127,000 miles in diameter. This wonderful
phenomenon was the subject of much investiga-
tion, particularly by Olbers, the great German
astronomer.

Heinrich Wilhelm Matthias Olbers was born
at Arbergen, a village near Bremen, October 11,
1758. His father was a clergyman who, in

124 A CENTURY'S PROGRESS IN ASTRONOMY.

addition to considerable mathematical powers,
was an enthusiastic lover of astronomy. At
the age of thirteen young Olbers became deeply
interested in that science. While taking an
evening walk in the month of August, he
observed the Pleiades, and determined to find
out to which constellation they belonged. He
therefore bought some books on astronomy,
along with a few charts of the sky, and he
began to study the science with much en-
thusiasm. He read every book he could lay
his hands on, and a few months sufficed to
make him acquainted with all the constellations.
In 1777, when in his nineteenth year,
Olbers entered the University of Gottingen
to study medicine, and at the same time
he learned much regarding mathematics and
astronomy from the mathematician Kaestner.
When twenty - one years of age he observed
the stars at Gottingen, and devised a method
of calculating the orbits of comets, the idea
coming to him while he was attending at the
bedside of a fellow -student who had taken ill.
"Although not made public until 1797," writes
Miss Clerke, "' Olbers' method' was then uni-
versally adopted, and is still regarded as the
most expeditious and convenient in cases where
absolute rigour is not required. By its intro-

COMETS. 125

duction, not only many a toilsome and thankless
hour was spared, but workers were multiplied
and encouraged in the pursuit of labours more
useful than attractive."

Towards the end of 1781 he returned to
Bremen, settled as a medical doctor, and con-
tinued in practice for about forty -one years.
But although he had adopted perhaps the most
toilsome profession, his love of science prevailed,
and night after night he explored the heavens
with untiring zeal. He never slept more than
four hours, and the upper part of his house in
the Sandgasse, in Bremen, was fitted up with
astronomical instruments. The largest telescope
which he possessed was a refractor 3f inches
in aperture. He remained in active practice
till 1823, when he retired, and was enabled
to devote more attention to his beloved science.
He died on March 2, 1840, at the advanced age
of eighty-one.

Miss Clerke says of Olbers, " Night after
night, during half a century and upwards, he
discovered, calculated, or observed the cometary
visitants of northern skies." He was the dis-
coverer of the comet of 1815, known as Gibers'
comet. It moves round the Sun in a period
of over seventy years, and returned to perihelion
in 1887, forty-seven years after the death of its

126 A CENTURY'S PROGRESS IN ASTRONOMY.

discoverer. The great comet of 1811 was the
subject of a memoir which Olbers published the
following year, and in which he originated the
" electrical repulsion " theory of comets' tails.
Even after the fulfilment of Halley's great
prediction, comets were still looked upon with
profound awe, and the popular fear regarding
them was still prevalent. Olbers, however,
showed that the tails of comets resulted from
purely natural causes. He regarded the Sun
as possessed of a repulsive as well as an
attractive force, and considered the tails to
be vapours repelled from the nucleus of the
comet by the Sun. He calculated that in
the comet of 1811 the particles of matter
expelled from the head reached the tail in
eleven minutes, with a velocity comparable to
that of light. The theory of electrical repul-
sion, since elaborated by other observers, is
now generally accepted among astronomers. No
other hypothesis represents in such a complete
manner the formation and growth of the lum-
inous appendages of the celestial bodies so
picturesquely called "pale -winged messengers"
as that put forward by the physician of
Bremen.

Some years after Olbers' famous theory was
given to the world, a great advance was made

COMETS. 127

in cometary astronomy by another great German
astronomer, his friend and pupil Encke. The
son of a Hamburg clergyman, Johann Franz
Encke was born in that city in 1791, and
died in 1865 at Spandau. After taking part
in the war against Napoleon, he was in 1822
appointed director of the Gotha Observatory,
being called to Berlin in 1825. In early life
he was the pupil of Olbers and Gauss, and
his investigations and discoveries formed an
epoch in astronomy. His most famous dis-
covery related to the little comet which bears
his name. The comet was discovered by J. L.
Pons (1761-1831) at Marseilles, although it had
previously been seen by Mechain and Caroline
Herschel. In 1819 Encke computed the orbit
of the comet, and boldly announced that it
would reappear in 1822, its period being about
3£ years, or 1208 days. In 1822 the comet,
true to Encke's prediction, returned to peri-
helion, and was observed at Paramatta in Aus-
tralia, the perihelion passage taking place within
three hours of the time predicted by Encke. As
Miss Clerke remarks, " The importance of this
event will be better understood when it is re-
membered that it was only the second instance
of the recognised return of a comet ; and that
it, moreover, established the existence of a new

128 A CENTURY'S PROGRESS IN ASTRONOMY.

class of celestial bodies, distinguished as comets
of short period."

In 1825 the comet was again observed by Valz,
passing perihelion on September 16, and in 1828
it was seen by Struve. Encke now made a very
remarkable discovery. Determining its period
with great accuracy, in 1832 he found that his
comet returned to perihelion two and a half
hours before the predicted time. As this re-
peatedly happened, Encke put forward the theory
that the acceleration was due to the existence
of a resisting medium in the neighbourhood of
the Sun, too rarefied to retard the planetary
motions, but quite dense enough to make the
comet's path smaller, and to eventually precipi-
tate it on the Sun. The theory was widely
accepted, but after 1868 the acceleration began
to decrease, diminishing by one-half; besides,
no other comet is thus accelerated, and the
hypothesis has accordingly been abandoned.

The second comet recognised as periodic was
that discovered on February 27, 1826, by an
Austrian officer, Wilhelm von Biela (1782-1856),
and ten days later by the French observer,
Gambart (1800-1836), both of whom, in com-
puting its orbit, noticed a remarkable similarity
to the orbits of comets which appeared in 1772
and 1805. Accordingly, they concluded it to

COMETS. 129

be periodic, with a period of between six and
seven years. The comet returned in 1832. In
1828 Olbers had published certain calculations
showing that portions of the comet would sweep
over the part of the Earth's orbit a month
later than the Earth itself. This gave rise
to a panic that the comet would destroy the
Earth, which did not subside till it was an-
nounced by Arago that the Earth and the
comet would at no time approach to within
fifty million miles of each other. The comet
returned again in the end of 1845. It was
kept well in view by astronomers in Europe
and America. On December 19, 1846, Hind
noticed that the comet was pear-shaped, and
ten days later it had divided in two. The
two comets returned again in 1852 and were
well observed ; but they were never seen again,
at least as comets. Their subsequent history
belongs to meteoric astronomy.

A comet discovered by Faye at Paris in 1843
was found to have a period of seven and a
half years. It has returned regularly since its
discovery, true to astronomical prediction. Its
motion was particularly investigated for traces
of a resisting medium, by Didrik Magnus Axel
Holler (1830-1896), director of the Lund Ob-
servatory, who reached a negative conclusion.

I

130 A CENTURY'S PROGRESS IN ASTRONOMY.

In 1835 Halley's comet returned to perihelion,
and was attentively studied by the most famous
astronomers of the age. It was particularly
studied by Sir John Herschel and by Bessel,
who assisted in developing Gibers' theory of
electrical repulsion. But the most brilliant
comet of the century was that which suddenly
appeared on February 28, 1843, in the vicinity
of the Sun. This great comet, whose centre
approached the Sun within 78,000 miles, rushed
past its perihelion at the speed of 366 miles a
second. The comet's tail reached the length of
200 millions of miles. The comet of 1843 was
however outshone, not in brilliance but as a
celestial spectacle, by the great comet discovered
on June 2, 1858, by Giovanni Battista Donati
(1826-1873) at Florence, and since known by
his name. It became visible to the naked eye
on August 19, and was telescopically observed
until March 4, 1859. There was abundance of
time, therefore, to study the comet, which was
exhaustively observed by G. P. Bond at Harvard.
His observations convinced him that the light
from Donati's comet was merely reflected sun-
shine, and this was generally accepted. Another
great comet appeared in 1861. Like that of
1843, its appearance was sudden, being observed
after sunset on June 30, 1861, when, says Miss

COMETS. 131

Clerke, " a golden yellow planetary disc, wrapt
in dense nebulosity, shone out while the June
twilight in these latitudes was still in its first
strength." On the same evening the Earth and
the Moon passed through the tail of the great
comet. The vast majority of people never knew
that such a phenomenon had taken place, and
even the astronomers only noticed a singular
phosphorescence in the sky — a proof of the
extreme tenuity of comets.

The first application of the spectroscope to
the light of comets was made by Donati in 1864.
The spectrum was found to consist of three bright
bands, but Donati was unable to identify them.
However, his observation gave the death-blow
to the theory that comets shone by reflected
light alone, for it implied the existence of
glowing gas in them. On the appearance in
1868 of the periodic comet discovered by
Friedrich August Theodor Winnecke (1835-1897),
the spectrum was examined by Huggins, who
identified the bright bands with the spectrum of
hydrocarbon. This was confirmed in regard to
Coggia's comet of 1874 by Huggins himself, and
also Bredikhine and Vogel. The hydrocarbon
spectrum is characteristic of comets, and has been
recognised in all those spectroscopically studied.

The time had now come for a more complete

132 A CENTURY'S PROGRESS IN ASTRONOMY.

theory of comets than that of Olbers. The
theory of electrical repulsion was developed in
1871 by Zollner, whose principle of investigation
is thus described by Miss Clerke : " The efficacy
of solar electrical repulsion relatively to solar
attraction grows as the size of the particle
diminishes." If the particle is small enough,
it will obey the repulsive, and not the attractive,
power of the Sun. Zollner considered that the
smallest particles of comets obeyed the repulsive
power, and thus formed the tails of comets. The
development of a complete cometary theory is
due, however, to the genius of a Russian astron-
omer. Theodor Alexandrovitch Bredikhine,
born in 1831 at NicolaiefF, was employed at
Moscow Observatory from 1857 to 1890, when
he was promoted to the position of director at
Pulkowa. He resigned in 1895, and spent his
last years in St Petersburg, where he died on
May 14, 1904. From the beginning of his
astronomical career he was devoted to the
study of comets and their tails, but it was the
appearance of Coggia's comet in 1874 which
marked the commencement of his most important
observations. In that year, on making certain
calculations regarding the hypothetical repulsive
force exerted by the Sun on various comets, he
reached the conclusion that the values repre-

COMETS. 133

senting the intensity of the repulsion fell into
three classes. This was the first hint of a
classification of cometary tails. Meanwhile he
carefully studied the tails of comets both from
direct observation and from drawings.

In 1877 he wrote: "I suspect that comets
are divisible into groups, for each of which the
repulsive force is perhaps the same." Subsequent
investigations led Bredikhine to divide the tails
of comets into three types. The first type con-
sists of long, straight tails, pointed directly away
from the Sun, represented by the tails of the
great comets of 1811, 1843, and 1861. In the
second type, represented by Donati's and Coggia's
comets, the tails, although pointed away from
the Sun, appear considerably curved. In the
third type the tails are, to quote Miss Clerke,
" short, strongly-bent, brushlike emanations, and
in bright comets seem to be only found in com-
bination with tails of the higher classes."

In 1879 Bredikhine fully developed his com-
etary theory. Assuming the reality of the re-
pulsive force, he concluded that to produce tails
of the first type, the repulsion requires to be
twelve times greater than the solar attraction ;
the production of tails of the second type
necessitates a repulsive force about equal to
gravity ; while the force producing third - type

134 A CENTURY'S PROGRESS IN ASTRONOMY.

tails has only one-fourth the power of gravita-
tion. It was concluded that the tails are formed
by particles of matter repelled from the comet
by the repulsive force of the Sun, and in tails
of the first type the velocity with which these
particles leave the body of the comet is four or
five miles a second. Bredikhine reached the
conclusion that the Sun's repulsive force is in-
variable, and that the different types of tails
are formed by the same force acting on different
elements. The numbers 12, 1, and J, are in-
versely proportional to the atomic weights of
hydrogen, hydrocarbon gas, and iron vapour.
Here, then, was the key to the mystery. Bre-
dikhine pointed out that in all probability the
first-type tails are formed of hydrogen, the second
of hydrocarbon, and the third of iron, with a
mixture of sodium and other elements.

Within a few years of the publication of Bre-
dikhine's theory, five bright comets made their
appearance, and there was abundant chance of
testing the theory spectroscopically. In 1882
Well's comet was particularly studied at Green-
wich by Maunder, who discerned a sodium-line
in its spectrum. The magnificent comet which
appeared in 1882 was spectroscopically studied
at Dunecht in Aberdeen shire by Ralph Gopeland
(1837-1905), Astronomer-Royal of Scotland, who

COMETS. 135

identified in its spectrum the prominent iron-lines
as well as the sodium-line. These observations
were certainly confirmatory of Bredikhine's theory.
It should also be stated, however, that several
comets have shown, in addition to the hydro-
carbon spectrum, that of reflected sunlight, which
proves that the light we receive from comets
is of a compound nature.

The comet which appeared in 1880 was an-
nounced by Benjamin Apthorp Gould (1824-1896)
to be a return of the great comet of 1843. Cal-
culations by Gould, Copeland, and Hind revealed
a close similarity between the elements of the
two orbits. Eventually it had to be admitted
that the comets were separate bodies travelling
in the same orbit. Then, two years later, the
great September comet of 1882 was found to
revolve in the same orbit as those of 1668, 1843,
and 1880. Four years later, another comet, dis-
covered in 1887, was found to move in the same
path.

Closely allied to this subject is the existence
of " comet families," demonstrated by Hoek of
Utrecht in 1865, and mentioned in our chapter
on the Outer Planets. These comets are found
to be dependent on the planets, Jupiter, Saturn,
Uranus, and Neptune, each possessing a comet-
group. Various theories have been advanced to

136 A CENTURY'S PROGRESS IN ASTRONOMY.

account for the existence of these groups. One
of these theories is that the comets have been
captured by the various planets, who have forced
them into their present orbits. A mathematical
study by Jean Pierre Octave Callandrean (1852-
1904) shows that the large number of comets
possessed by the various planets may be ex-
plained by the disintegration of large comets into
small ones. The capture theory, it must be re-
membered, is purely hypothetical, and must not
be regarded as anything but a theory. All that
we really know is the existence of comet>-families,
and of comets moving in the same orbits.

The first photograph of a comet was that of
Donati's, taken in 1858 by Bond. In 1881 Teb-
butt's comet was photographed in England by
Huggins, and in America by Henry Draper
(1837-1882), while in 1882 Gill secured excellent
photographs of the great September comet. The
first photographic discovery of a comet was made
by Barnard in 1892. Since then photography
has been much used in cometary astronomy. No
bright comets have appeared since 1882, — if we
except the comet of 1901, only seen in the
southern hemisphere, — although several have
been just visible to the naked eye, among them
Swift's comet of 1892 and Perrine's in the
autumn of 1902. Telescopic comets, however,

COMETS. 137

are very numerous, and a year never passes
without one or more being discovered. The
ordinary periodic comets, such as Encke's, Faye's,
and others, are very faint, and are becoming
fainter at each return — a clear proof that comets
die, as Kepler said three centuries ago. This
brings us to the subject of the next chapter,
Meteoric Astronomy.

CHAPTER VIII.

METEORS.

THERE is no more interesting chapter in the
history of astronomy than that relating to
meteors. A hundred years ago shooting -stars
were not considered to be astronomical phe-
nomena. They were supposed to be merely
inflammable vapours which caught fire in the
upper regions of our atmosphere, although both
Halley and the scientist Ernst Chladni (1756-
1827) had notions of their celestial origin. For
thirty -three years after the beginning of the
century, however, nothing was heard of meteoric
astronomy, nor was the subject considered as
part of the astronomer's labours.

A great meteoric shower took place on
the night of November 12 and morning of
November 13, 1833. The shower was probably
the grandest ever witnessed, the shooting-stars
being literally innumerable. The display was
best observed in America, and was attentively

METEORS. 139

watched by Denison Olmsted (1791-1859), Pro-
fessor of Mathematics at Yale, and by the Ameri-
can physicist, A. C. Twining (1801-1884). These
investigators discovered that all the meteors
which fell during the great shower seemed to
come from the same part of the celestial vault.
In other words, their paths, when traced back,
were found to converge to a point near the star
y Leonis. This observation gave the death-
blow to the theory of their terrestrial origin.
The point known as the "radiant" was clearly
a point independent of the Earth. Olmsted also
recognised the fact that the shower had taken
place in the previous year, and he regarded it as
produced by a swarm of particles moving round
the Sun in a period of 182 days. Soon after this
it was noticed that the phenomenon took place
in 1834 and subsequent years with gradually
decreasing intensity. It was then remembered
that Humboldt had observed in November 1799
a very brilliant shower, and accordingly Olbers
suggested that another shower might be seen
in 1867.

The falling stars of August were next proved
by Adolphe Quetelet (1791-1874) to form another
meteoric system ; and accordingly the theory of
Olmsted that the November meteors moved
round the Sun in 182 days had to be abandoned,

140 A CENTURY'S PROGRESS IN ASTRONOMY.

for, says Miss Clerke, " If it would be a violation
of probability to attribute to one such agglomera-
tion a period of an exact year or sub -multiple
of a year, it would be plainly absurd to suppose
the movements of two or more regulated by such
highly artificial conditions." Accordingly Erman
suggested in 1839 the theory that meteors re-
volved in closed rings, intersecting the terrestrial
orbit ; and that when the Earth crossed through
the point of intersection, it met some members
of the swarm. The subject now remained in
abeyance for thirty-four years, if we except some
wonderful ideas put forward in 1861 by Daniel
Kirkwood (1813-1896), an American astronomer,
who stated his belief in the disintegration of
comets into meteors ; but little attention was
paid to his opinions. In 1864 the subject was
taken up by Hubert Anson Newton (1830-
1896), Professor at Yale, who undertook a search
through ancient records for the thirty-three-year
period of the Leonids or November meteors. His
search was highly successful, and having demon-
strated the existence of the period, Newton set
himself to determine the orbit. He indicated
five possible orbits for the swarm, ranging from
33 years to 354J days. Newton was unable to
solve the question mathematically ; but here
Adams, the discoverer of Neptune, came to the

METEORS. 141

rescue, and demonstrated that the period of 33|-
years was alone possible, and that the others
were untenable. These investigations, completed
in March 1867, proved the existence of a great
meteoric orbit extending to the orbit of Uranus.
Meanwhile Newton had predicted a meteoric
shower on the evening of November 13 and
morning of November 14, 1866. His prediction
was fulfilled. The shower was inferior to that of
1833, but was still a magnificent spectacle. Sir
Robert Ball, then employed at Lord Rosse's
Observatory, observed the shower, and records
the impossibility of counting the meteors. This
great shower attracted the attention of astron-
omers all over the world to the study of meteors.
Meanwhile Schiaparelli had been working at
the subject for some time, and in four letters
addressed to Secchi, towards the end of 1866, he
showed that meteors were members of the Solar
System, possessed of a greater velocity than that
of the Earth, and travelling in orbits resembling
those of comets, in the fact that they moved in no
particular plane, and that their motion was both
direct and retrograde. Schiaparelli computed
the orbit of the Perseids or August meteors,
and was astonished to find it identical with the
comet of August 1862. This was a proof of the
connection between these two apparently widely

142 A CENTURY'S PROGRESS IN ASTRONOMY.

different types of celestial bodies. Early in 1867
Sehiaparelli found that Le Verrier's elements for
the orbit of the Leonids were identical with
those of the comet of 1866, discovered by Ernst
Tempel (1821-1889). Peters of Altona had
meanwhile reached the same conclusion ; while
Edmund Weiss (born 1837) of Vienna pointed
out the similarity of the orbit of a star-shower
on April 20 and that of the comet of 1861.
He also drew attention, independently of Galle
and D' Arrest, to the close connection between
the orbits of the lost Biela's comet and the
Andromedid meteors of November.

All doubt as to the connection of comets and
meteors was removed by the great shower on
November 27, 1872. Biela's lost comet was due
at perihelion in 1872, and although searched for
was not observed ; but when the Earth crossed
its orbit, a great meteoric shower took place.
"It became evident," says Miss Clerke, "that
Biela's comet was shedding over us the pulver-
ised products of its disintegration." The shower
was little inferior to that of 1866. Meanwhile
Ernst Klinkerfues (1827-1884), Professor at
Gottingen, believing that Biela's comet itself had
encountered the Earth, telegraphed to Norman
Robert Pogson (1829-1891), Government astron-
omer at Madras, to search for the comet in the

METEORS. 143

opposite region of the sky. Pogson did observe
a comet, but certainly not Biela's, although prob-
ably another fragment of the missing body.

The theory of the actual disintegration of
comets was enunciated by Schiaparelli in 1873,
and developed in his work ' Le Stelle Cadenti/
He was led to regard comets as cosmical clouds
formed in space by " the local concentration of
celestial matter." He then remarks that a cos-
mical cloud seldom penetrates to the interior of
the Solar System, " unless it has been trans-
formed into a parabolic current," which may
occupy years, or centuries, in passing its
perihelion, " forming in space a river, whose
transverse dimensions are very small with respect
to its length : of such currents, those which are
encountered by the earth in its annual motion
are rendered visible to us under the form of
showers of meteors diverging from a certain
radiant."

Schiaparelli next pointed out that when the
current of meteors encounters a planet, the re-
sulting perturbations cause some of the meteoric
bodies to move in separate orbits, forming the
bolides and aerolites which fall from the sky at
intervals. " The term falling stars" he says,
" expresses simply and precisely the truth
respecting them. These bodies have the same

144 A CENTURY'S PROGRESS IN ASTRONOMY.

relation to comets that the small planets
between Mars and Jupiter have to the larger
planets." In the third chapter of his ' Le
Stelle Cadenti' he explicitly states that
"the meteoric currents are the products of
the dissolution of comets, and consist of minute
particles which certain comets have abandoned
along their orbits, by reason of the disintegrat-
ing force which the Sun and planets exert on
the rare materials of which they are composed."
In 1878 Alexander Stewart Herschel (born
1836), son of Sir John Herschel, and a famous
meteoric observer, published a list of known or
suspected coincidences of meteoric and cometary
orbits, amounting to seventy -six. Meanwhile
much progress has since been made in the
observation of meteoric showers and the deter-
mination of their radiant points. In this branch
of astronomy, by far the greatest name is that
of William Frederick Denning, the self-made
English astronomer. Born at Redpost, in
Somerset, in 1848, his career of meteoric ob-
servation commenced in 1866. For the past
forty years he has attentively devoted him-
self to the observation of meteors. From 1872
to 1903 he determined the radiant points of
no fewer than 1179 meteoric showers. In
addition to this, he published, in 1899, a

METEORS. 145

catalogue of meteoric radiants, containing 436*7 ;
and he has carefully studied the remarkable
objects known as fireballs or " sporadic meteors."
He has occasionally been able to trace a con-
nection between fireballs and weak meteoric
showers, but he concludes that they " must
either be merely single sporadic bodies, or else
the survivors of some meteor group, nearly
exhausted by the waste of its material during
many past ages." All of Denning's meteoric
work has been done in his spare time, for it
must be borne in mind that he pursues the
profession of accountant in Bristol, and that
only his leisure hours have been devoted to
the science of astronomy. His researches have
been entirely conducted with the unaided eye.
His only instrument is a perfectly straight
wand, which he uses as a help and corrective
to the eye in ascribing the paths of the meteors.
Thanks to the laborious work of this able
English astronomer, the observation of meteors
is now a scientific branch of astronomy. In
the words of Maunder, " for six thousand years
men stared at meteors and learned nothing, for
sixty years they have studied them and learned
much, and half of what we know has been
taught us in half that time by the efforts of
a single observer."

K

146 A CENTURY'S PROGRESS IN ASTRONOMY.

Further meteoric showers from Biela's comet
were seen in 1885 and 1892. The Leonid
shower was confidently predicted for 1899, in
accordance with the thirty -three -year period,
but the great display did not come off, either
in 1899 or 1900. In 1901 there was a certain
weak shower observed in America ; and similar
displays took place in 1903 and 1904. Many
explanations have been given as to the failure
of the shower, the most probable idea being
that the attraction of Jupiter diverted the
meteors from their course.

Denning's observations on meteors resulted,
as early as 1877, in the discovery of so-called
"stationary radiants." The radiant-point of
a long enduring shower usually exhibits an
apparent motion, resulting from the combined
orbital motions of the Earth and the meteors ;
but Denning found that in some cases the
shower, though lasting for months, persistently
exhibited the same radiant-point, implying that
the motion of the Earth must be insignificant
compared with that of the meteors, computed
by Ranyard at 880 miles per second. The dif-
ficulty of admitting so great a velocity led
the French astronomer, Francois Felix Tisserand
(1845-1896), to doubt the existence of these
stationary radiants ; but the fact of their

METEORS. 147

existence cannot be doubted, although no really
satisfactory explanation has been offered.

Another type of meteors comprises the bodies
termed respectively as bolides, uranoliths, and
aerolites, — stones which fall to the Earth from
the sky. In 1800 the French Academy declared
the accounts of stones having fallen from the
heavens to be absolutely untrue. Three years
later an aerolite fell at Laigle, in the Depart-
ment of Orne, on April 26, 1803, attended by
a terrific explosion. In the words of Flammarion,
" Numerous witnesses affirmed that some minutes
after the appearance of a great bolide, moving
from south-east to north-east, and which had
been perceived at Alen9on, Caen, and Falaise,
a fearful explosion, followed by detonations like
the report of cannon and the fire of musketry,
proceeded from an isolated black cloud in a
very clear sky. A great number of meteoric
stones were then precipitated on the surface of
the ground, where they were collected, still
smoking, over an extent of country which
measured no less than seven miles in length."

Some aerolites, instead of being shattered into
fragments, have been observed to fall to the
Earth intact, and bury themselves in the ground.
Numerous instances have been observed during
the last century, and masses of meteoric stones

148 A CENTURY'S PROGRESS IN ASTRONOMY.

have been found in positions which clearly in-
dicate that they must have fallen from the sky.
Chemists have made analyses of the elements in
these remarkable bodies, and have found them
to contain iron, magnesium, silicon, oxygen,
nickel, cobalt, tin, copper, &c. The spectrum
of these aerolites, raised to incandescence, has
been studied by Vogel and by the Swedish
observer, Bernhard Hasselberg (born 1848), who
detected the presence of hydrocarbons, which are
also present in cometary spectra.

When the existence of aerolites as celestial
bodies was first recognised, Laplace suggested
that they had been ejected from volcanoes on
the Moon. This theory, although supported
by Olbers and other astronomers, was soon
rejected. Next, it was suggested that they
were ejected from the Sun, and Proctor believed
them to come from the giant planets. A very
detailed discussion of the subject is to be found
in Ball's ' Story of the Heavens' (1886), in which
he expresses views in harmony with those of
the Austrian physicist Tschermak. Ball demon-
strated that the meteors which fall to the Earth
cannot have come from any other planet, nor
from the Sun. Accordingly, he concluded that
they were originally ejected by the volcanoes
of the Earth many ages ago, when they were

METEORS. 149

active enough to throw up pieces of matter with
a velocity great enough to carry them away
from the Earth altogether. Such meteors would,
however, intersect the terrestrial orbit at each
revolution.

The alternative theory to this, supported by
Schiaparelli and Lockyer, is that the aerolites
are merely larger members of the meteor- swarms,
which have been deflected from their paths. The
chief objection to this theory is the absence
of connection between the meteoric showers and
the falls of aerolites and bolides. Only on one
occasion was a meteoric stone observed to fall
during a shower. On November 27, 1885, during
the shower of Andromedid meteors from Biela's
comet, a large bolide, weighing more than eight
pounds, fell at Mazapil, in Mexico. This, how-
ever, was the only case hitherto observed ; and
it may have been merely a coincidence.

CHAPTEE IX.

THE STARS.

THE most remarkable progress in astronomy
during the past century has been in the de-
partment of sidereal science, or the study of
the Suns of space, observed for their own sakes,
and not merely for the purpose of determining
the positions of the Sun and Moon, and to assist
navigation. Thanks to Herschel, the nineteenth
century witnessed the steady development of
stellar astronomy, combined with many import-
ant discoveries and investigations.

The one pre - Herschelian problem in sidereal
astronomy was the distance of the stars. Owing
to its bearing on the Copernican theory, the
problem was attacked by the astronomers of the
seventeenth and eighteenth centuries. Herschel
made numerous attempts to detect the parallax
of the brighter stars, but failed. Meanwhile
there had been many illusions. Piazzi believed
that his instruments — which in reality were

THE STARS. 151

worn out and unfit for use — had revealed
parallaxes in Sirius, Aldebaran, Procyon, and
Vega ; Calandrelli, another Italian, and John
Brinkley (1763-1835), Astronomer-Royal of Ire-
land, were similarly deluded; and in 1821 it was
shown by Friedrich Georg Wilhelm Struve (1793-
1864), the great German astronomer, that no
instruments then in use could possibly be suc-
cessful in measuring the stellar parallax. A few
years later, however, Fraunhofer brought the
refractor to a degree of perfection surpassing all
previous efforts. In 1829 he mounted for the ob-
servatory at Konigsberg a heliometer, the object-
glass of which was divided in two, and capable
of very accurate measurements. This heliometer
eventually revealed the parallax of the stars in
the able hands of Friedrich Wilhelm Bessel.

Friedrich Wilhelm Bessel was born at Minden,
on the Weser, south-west of Hanover, on July
22, 1784. His father was an obscure Govern-
ment official, unable to provide a university
education for his son. Bessel's love of figures,
together with an aversion to Latin, led him
to pursue a commercial career. At the age of
fourteen, therefore, he entered as an apprenticed
clerk the business of Kuhlenkamp & Sons, in
Bremen. He was not content, however, to
remain in that humble position. His great

152 A CENTURY'S PROGRESS IN ASTRONOMY.

ambition was to become supercargo on one of
the trading expeditions sent to China ; and so
he learned English, Spanish, and geography.
But he never became a supercargo. In order
to be fully equipped for such a position, he
determined to learn how to take observations
at sea, and his acquaintance with observation
aroused a desire to study astronomy. He con-
structed for himself a sextant, and by means of
this, along with a common clock, he determined
the longitude of Bremen.

Such enthusiasm could not be long without
its reward. For several years Bessel remained
a clerk, and the hours devoted to study were
those spared from sleep. He studied the works
of Bode, Von Zach, Lalande, and Laplace, and
in two years was able to compute the orbits of
comets by means of mathematics. From some
observations of Halley's comet at its appearance
in 1607, Bessel calculated its orbit, and for-
warded the calculation to Olbers, then the
greatest authority on cometary astronomy.
Olbers was delighted at this work, and he
sent the results to Von Zach, who published
them. The self-taught young astronomer had
accomplished a piece of work which fifteen years
before had taxed the skill and patience of the
French Academy of Sciences.

THE STARS. 153

In 1805, Harding, Schroter's assistant at
Lilienthal, resigned his position for a more
promising one at Gottingen. Gibers procured
for Bessel the offer of the vacant post, which the
latter accepted. At Lilienthal Bessel received
his training as a practical astronomer. He re-
mained in Schroter's observatory until 1809.
Although only twenty -five years of age, he
had become so well known in Germany that in
that year he was appointed Professor of As-
tronomy in the University of Konigsberg, and
was chosen to superintend the erection of the
new observatory there. Within a few years
a clerk in a commercial office had worked his
way from obscurity to fame.

In 1813 the Konigsberg Observatory was
completed, and here Bessel worked for thirty-
three years, until his death, on March 17, 1846.
It was only about ten years before his death that
he commenced his search for the stellar parallax,
with the aid of Fraunhofer's magnificent helio-
meter. He determined to make a series of meas-
ures on a small double star of the fifth mag-
nitude in the constellation Cygnus, named 61
Cygni, the large proper motion of which led him
to suspect its proximity to the Solar System.
From August 1837 to September 1838 he made
observations on 61 Cygni, and he found that

154 A CENTURY'S PROGRESS IN ASTRONOMY.

there was an annual displacement which could
only be attributed to parallax. In order to have
no mistake, he made another year's observa-
tions, which confirmed the results he arrived at
previously, and all doubt was removed by a
third series. The resulting parallax was 0*3483",
corresponding to a distance of 600,000 times
the Earth's distance from the Sun. This was
confirmed some years later by C. A. F. Peters
at Pulkowa, and still later by Otto Struve, who
estimated the distance at forty billions of miles.
Meanwhile, F. G. W. Struve, working at Pul-
kowa, found a parallax of 0*2613" for Vega,
but this was afterwards found to be consider-
ably in error. Accordingly, Struve does not
rank with Bessel as a successful measurer of star-
distance. But independently of Bessel, another
accurate measure had been made by Thomas
Henderson, the great Scottish astronomer.

Born in Dundee in 1798, Thomas Henderson
was the youngest of five children of a hard-
working tradesman. After education in his
native town he went to Edinburgh, where he
worked for years as an advocate's clerk, pur-
suing studies in astronomy as a recreation from
his boyhood. In 1831 he had become so well
known, that he received the appointment of
Astronomer -Royal at the new observatory at

THE STARS. 155

the Cape of Good Hope. But the climate of
South Africa did not suit his health, and after
a year he returned to Scotland. In 1834 he
became Professor of Astronomy in the Uni-
versity of Edinburgh, and Astronomer-Royal of
Scotland, which position he held till his death
on November 23, 1844, at the early age of
forty-six.

During a year's work at the Cape, Hender-
son undertook a series of observations on the
bright southern star, a Centauri, with a view
to determining its parallax. These observations
were made in 1832 and 1833, but were not
reduced until Henderson's return to Scotland.
At length, on January 3, 1839, he announced
to the Royal Astronomical Society that he
had succeeded in measuring the parallax of
a Centauri, which he determined as about one
second of arc, corresponding to a distance of
about twenty billions of miles. This result
was confirmed by the observations of Thomas
Maclear (1794-1879), his successor at the Cape,
and by those of later observers, notably Sir
David Gill, who has reduced the parallax to
075".

Other determinations of stellar parallax, some
genuine and others illusory, were made soon after
these successful observations. C. A. F. Peters

156 A CENTURY'S PROGRESS IN ASTRONOMY.

and Otto Struve at Pulkowa were among the
most famous parallax-hunters in the middle of the
century. One of the most successful searchers
after parallax was the German astronomer Fried-
rich Briinnow (1821-1891), who was employed
from 1865 to 1874 as Astronomer-Royal of Ire-
land. He determined the parallax of Vega as
0*13", and this was confirmed in 1886 by Hall
at Washington : while he measured the parallax
of the star Groombridge 1830, which turned out
to be 0'09". He resigned his post in 1874, and
his successor at Dublin Observatory proved to
be his successor also in this branch of astronomy.
Robert Stawell Ball, born in Dublin in 1840,
was astronomer to Lord Rosse in 1865 and 1866,
and became in 1874 Astronomer-Royal of Ire-
land in succession to Briinnow, a position which
he filled until his appointment in 1892 as Pro-
fessor of Astronomy at Cambridge, and director
of the observatory there. During his term of
office in Dublin he undertook, in 1881, a
" sweeping search " for large parallaxes, thereby
disproving certain ideas as to the proximity to
the Earth of red and temporary stars; while
he also determined the parallax of the star
1618 Groombridge.

But the greatest extension of our knowledge
of stellar distances, in recent years, is due to a

THE STARS. 15*7

Scottish astronomer, who has maintained the
reputation of Scotland, and also of the Cape
Observatory, in this line of research. Born in
Aberdeen in 1843, David Gill directed Lord
Lindsay's private observatory at Dunecht, in
Aberdeenshire, from 1876 to 1879. In the latter
year he succeeded Edward James Stone (1831-
1897) as Astronomer-Royal at the Cape, a posi-
tion which he has since filled with conspicuous
ability. From 1881 he has been engaged in the
hunt for parallax. In conjunction with William
Lewis Elkin (born 1855), now director of Yale
College Observatory, he determined the par-
allaxes of nine stars with the aid of Lord
Lindsay's heliometer. In 1887, with a larger
instrument, he resumed the search, while Elkin
worked in co-operation with him, but at Yale
Observatory, where he undertook the measure-
ment of the parallaxes of northern stars. He
fixed in 1888 an average parallax for first-
magnitude stars, which was determined at 0*089",
corresponding to a journey for light of thirty-
six years.

Most of the successful determinations of par-
allax have been made by the " relative " method —
that is, the determination of the displacement
of a star in reference to another star, assumed
to be situated at an immeasurable distance.

158 A CENTURY'S PROGRESS IN ASTRONOMY.

The method of absolute parallax, on the other
hand, — the star's displacement in right ascension
and decimation, — has been seldom used, owing
to the laborious reduction which has to be gone
through before the result can be reached. In
1885, however, a series of observations were
undertaken at Leyden by Jacobus Cornelius
Kapteyn (born 1851), who determined by the
absolute method the parallaxes of fifteen north-
ern stars.

The first application of photography to the
problem was due to the zeal and energy of
Charles Pritchard (1808-1893), Professor of
Astronomy at Oxford, who determined by this
method the parallax of 61 Cygni, which he
announced in 1886 to be 0*438", in agreement
with Ball's determination. He also determined
the average parallax of second-magnitude stars,
which came out as 0'056". Since the time of
Pritchard's observations various other more or
less satisfactory determinations of parallax have
been made. Few of the parallax determinations
are probably very accurate, and none exact ; but
an idea of the difficulty of the measurement may
be gathered from the remark of an American
writer, Mr G. P. Serviss, that the displacement
" is about equal to the apparent distance between
the heads of two pins, placed an inch apart, and

THE STARS. 159

viewed from a distance of a hundred and eighty
miles."

Closely allied to the question of parallax is
the determination of the exact positions of the
stars and the formation of star-catalogues. In
this branch, too, much is due to the genius of
Bessel. The observations of Bradley at Green-
wich from 1750 to 1762 were reduced by Bessel
into the form of a catalogue, which was published
in 1818, with the title of 'Fundamenta Astro-
nomiae.' During the years 1821 to 1823 Bessel
took 75,011 observations, by which he brought
up the number of accurately known stars to
50,000. At the same time notable catalogues
had been constructed, particularly by the Eng-
lish astronomer, Francis Baily (1774 - 1844),
and by Giovanni Santini (1786-1877), director
of the observatory at Padua ; but Bessel's suc-
cessor in this branch of research was Friedrich
Wilhelm August Argelander (1799 - 1875). In
1821 he became assistant to Bessel at Konigs-
berg, in 1823 director of the Observatory at Abo,
in Finland, and in 1837 of that at Bonn. Here
he commenced in 1852 the great * Bonn Durch-
musterung,' a catalogue and atlas of 324,198
stars visible in the northern hemisphere. The
great catalogue was published in 1863. After
Argelander's death it was extended so as to

160 A CENTURY'S PROGRESS IN ASTRONOMY.

include 133,659 stars in the southern hemisphere,
by his assistant Eduard Schonfeld (1828-1891),
who succeeded him in 1875 as director of Bonn
Observatory, where he died in 1891. Meanwhile
a greater undertaking was commenced in 1865 by
the Astronomische Gesellschaft. This was the
co-operation of thirteen observatories in Europe
and America for the exact determination of the
places of 100,000 of Argelander's stars.

In the southern hemisphere, working at Cor-
dova in Argentina, was the great American
astronomer, Gould, whose c Uranometria Argen-
tina/ published in 1879, gives the magnitudes
of 8198 stars, and whose Argentine General
Catalogue, containing reference of 32,448 stars,
was published in 1886. The late Radcliffe ob-
server, Stone, published a useful catalogue in
1880 from his observations at the Cape.

The application of photography to the work
of star - charting dates from 1882, when Gill
photographed the comet of 1882, and was struck
with the distinctness of the stars on the back-
ground. For some time he had contemplated the
extension of the * Durchmusterung,' from the
point where Schonfeld left it, to the southern
pole, and the idea struck him to utilise phot-
ography for the purpose. In 1885, accordingly,
Gill commenced work, and in four years all the

THE STARS. 161

photographs were taken. The reduction of the
observations into the form of a catalogue was
spontaneously undertaken by the great Dutch
astronomer, Kapteyn, who was occupied with the
work for fourteen years, until in 1900 the great
catalogue, known as the ' Cape Photographic
Durchmusterung,' was completed. Half a million
stars are represented on the plates taken at the
Cape.

By the time the ' Durchmusterung ' was com-
pleted, a greater undertaking was in progress.
Paul and Prosper Henry, astronomers at the
Paris Observatory, when engaged in continuing
Chacornac's ecliptic charts, applied photography
to their work, and found it very successful.
Accordingly Gill's proposal, on June 4, 1886, of
an International Congress of Astronomers, to
undertake a photographic survey of the heavens,
was enthusiastically received by the French
astronomers. The Congress met at Paris in
1887, under the presidentship of Amedee Mou-
chez (1821-1892), director of the Paris Observa-
tory, fifty -six astronomers of all nations being
present. The Congress resolved to construct a
Photographic Chart, and a Catalogue, the former
containing twenty million stars, the latter a
million and a quarter. Meetings were held in
Paris in 1891, 1893, 1896, and 1900 to super-

L

162 A CENTURY'S PROGRESS IN ASTRONOMY.

intend the progress of the work, which is now
(1906) well advanced towards completion.

A unique star catalogue is in course of prep-
aration by the Scottish astronomer, William
Peck (born 1862), astronomer to the City of
Edinburgh since 1889. Mr Peck's catalogue is
accompanied by a series of charts. His star-
magnitudes are those of all famous catalogues
reduced to a standard scale. This catalogue,
the result of more than fifteen years' work, will
be an important addition to the many valuable
works of the kind already in existence, and will
further increase the already great reputation of
Scotsmen in practical astronomy.

The determination of the proper motions of
the stars is another important branch of practi-
cal astronomy in which much progress has been
made since the time of Herschel. Stars with
much larger proper motions than those of the
first magnitude have been discovered. For
many years the small sixth -magnitude star in
Ursa Major, 1830 Groombridge, was supposed
to be the swiftest of the stars, and was named
by Newcomb the " runaway star." But in
1897, on examining the plates of the 'Cape
Durchmusterung/ Kapteyn discovered a still
swifter star of the eighth magnitude, situated
in the southern constellation, Pictor. The rate

THE STABS. 163

of its motion is over eight seconds of arc
yearly ; and an idea of the vast distance of the
stars may be obtained by the statement that it
would take 200 years for the star — known as
Gould's Cordova Zones, V Hour 243 — to move
over a space equal to the moon's diameter.
Important observations have been made on the
stellar motions, and on their bearing on the
structure of the Universe, by various astron-
omers, including J. C. Kapteyn and Ludwig
Struve (born 1858), son of Otto Struve; but
these must be reserved for a later chapter.

Richard Anthony Proctor, born at Chelsea, in
London, in 1837, graduted at Cambridge in 1860.
For the next twenty -eight years he earned his
living by publishing many volumes on astronomy,
popular and technical, fifty-seven having appeared
at the time of his death, which took place at
New York on September 12, 1888. Notwith-
standing the vast amount of work bestowed on
his books, his original investigations were per-
manent contributions to astronomical science.
In 1870 he undertook to chart the directions
and amounts of 1600 proper motions. While
engaged on this work, it occurred to him that
it would be " desirable and useful to search for
subordinate laws of motion." He found, from
the laborious process of charting, that five of

164 A CENTURY'S PROGRESS IN ASTRONOMY.

the seven stars of the Plough had a motion in
common — that is to say, were moving in the
same direction at the same rate. This phe-
nomenon was termed by Proctor " star -drift."
He also recognised other instances of star-drift
in other portions of the heavens.

The subject was soon afterwards taken up
by the French astronomer, Camille Flammarion.
Born in 1842 at Montigny-le-Roi, in Haute
Marne, Flammarion was appointed assistant
to Le Verrier in 1858, but gave up his post in
1862. Employed successively at the Bureau des
Longitudes, and as editor of scientific papers,
he founded in 1882 his private observatory at
Juvisy-sur-Orge, where he has since continued
his investigations.

Following up Proctor's discovery of star-drift,
Flammarion drew charts of proper motions. He
demonstrated the " common proper motion " of
Regulus and an eighth-magnitude star, Lalande
19,749, from a comparison of his measures in
1877 with those of Christian Mayer a century
previously ; while he discovered many other
instances. His reflections on these motions, as
given in his 'Popular Astronomy/ are worthy
of reproduction : " Such are the stupendous
motions which carry every sun, every system,
every world, all life, and all destiny in all

THE STARS. 165

directions of the infinite immensity, through the
boundless, bottomless abyss ; in a void for ever
open, ever yawning, ever black, and ever un-
fathomable ; during an eternity, without days,
without years, without centuries, or measures.
Such is the aspect, grand, splendid, and sublime,
of the universe which flies through space before
the dazzled and stupefied gaze of the terrestrial
astronomer, born to-day to die to-morrow, on a
globule lost in the infinite night."

Measures of proper motion only enable us to
determine the motion of stars across the line
of sight. They do not tell us whether the
star is advancing or receding. Here, however,
the spectroscope comes to our aid by means of
Doppler's principle, described in the chapter on
the Sun. It occurred to Huggins that, by
observing the displacement of the lines in the
spectra of the stars, he could determine their
motion in the line of sight. His first results
were announced in 1868. In the case of
Sirius, the displacement of the line marked F
was believed to indicate a velocity of recess-
ion of 29 miles a second. Some time later
Huggins announced that Betelgeux, Eigel,
Castor, and Regulus were retreating, while
Arcturus, Pollux, Vega, and Deneb were ap-
proaching. Soon after this successful work the

166 A CENTURY'S PROGRESS IN ASTRONOMY.

subject was taken up by Maunder at Greenwich
and by Vogel at Bothkamp ; but the delicacy
of the measurements prevented satisfactory re-
sults from being reached through visual observa-
tions, and accordingly the measurements were
very discordant.

In 1887 H. C. Vogel, working at Potsdam
Astrophysical Observatory, applied photography
to the measurement of radial motion. Assisted
by Julius Scheiner (born 1858), he determined
the radial motions of fifty-one bright stars by
photographing the stellar spectra and measur-
ing the photographs. Vogel found 10 miles a
second to be the average velocity of stars in
the line of sight, the tendency of the eye being
to exaggerate the displacements. The swiftest
of the stars measured by Vogel proved to be
Aldebaran, with a velocity of recession of 30
miles a second. Since 1892 the subject has
been pursued by Vogel himself with the new
30 -inch refractor at Potsdam, by Campbell at
the Lick Observatory, B^lopolsky at Pulkowa,
and other observers. Towards the end of 1896
Campbell undertook, with the 36 - inch Lick
refractor, a series of measures on radial motion,
and many important discoveries were made.
These, however, must be reserved for the chapter
dealing with double stars.

THE STABS. 167

Herschel's great discovery, from the apparent
motions of the stars, of the movement of the
Solar System was not accepted by the next
generation of astronomers. Bessel declared in
1818 that there was absolutely no evidence to
show that the Sun was moving towards Hercules.
Even Sir John Herschel rejected his father's
views, although some confirmatory results had
been reached by Gauss. At length, in 1837,
Argelander, in a memorable paper, based on his
observations at Abo, in Finland, attacked the
problem, and demonstrated, from a discussion of
the motions of 390 stars, quite independently of
Herschel's work, that the Solar System was
moving towards Hercules. This was confirmed
in 1841 by Otto Struve, in 1847 by Thomas
Galloway, and in 1859 and 1863 by Airy and
Edwin Dunkin (1821-1898), assistant at Green-
wich Observatory.

Meanwhile, in 1886, Arthur Auwers, perma-
nent Secretary of the Berlin Academy of Sciences,
completed the re-reduction of Bradley's observa-
tions at Greenwich, and brought out 300 reliable
proper motions, which were utilised by Ludwig
Struve, whose investigation removed the solar
apex from Hercules to the neighbouring con-
stellation Lyra : this slight change was con-
firmed by Oscar Strumpe, of Bonn, and Lewis

168 A CENTURY'S PROGRESS IN ASTRONOMY.

Boss (born 1847), director of the Observatory
at Albany, New York. An investigation by
Newcomb fully confirmed the previous results.
In 1900, 1901, and 1902 Kapteyn made three
distinct investigations on the solar motion, and
still further confirmed the previous investiga-
tions.

These investigations are fully confirmed by
the application to the question of Doppler's
principle of measuring radial motion. The
spectroscopic researches of Campbell at the
Lick Observatory place the solar apex very
near the position assigned to it by Newcomb
and Kapteyn. Campbell finds the solar velocity
to be about 12 miles a second, and Kapteyn
thinks a velocity of about 11 miles a second is
" the most probable value that can at present
be adopted."

CHAPTER X.

THE LIGHT OF THE STABS.

" THAT a science of stellar chemistry should not
only have become possible, but should already
have made material advances, is assuredly one
of the most amazing features in the swift pro-
gress of knowledge our age has witnessed/' So
writes Miss Agnes Mary Clerke, the historian
of modern astronomy. As long ago as 1823
Fraunhofer observed the spectra of the brighter
stars, and gathered the first hint of the group-
ing of the stars into three classes. Then, after
Fraunhofer's death, the subject lay in abeyance
for thirty-seven years. At length, in 1860, on
Kirchhoffs explanation of the Fraunhofer lines,
the study of stellar spectra was inaugurated at
Florence by Donati, who carefully fixed the posi-
tions of the more important lines. His instru-
mental means, however, were very limited, and
his observations were not successful. In 1862
Rutherford, in New York, commenced the study

170 A CENTURY'S PROGRESS IN ASTRONOMY.

of stellar spectra, but shortly afterwards turned
his attention to astronomical photography. The
actual founders of stellar spectroscopy were the
eminent Italian observer, Angelo Secchi, and the
illustrious Englishman, William Huggins.

Angelo Secchi was born in 1818 at Reggio, in
the Emilia. Educated in the Collegio Romano,
he was ordained priest in 1847, but his love of
science, and particularly astronomy, dates from
the beginning of his career. In 1849 he suc-
ceeded Di Vico as director of the Observatory
of the Collegio Romano. This post he filled
with conspicuous ability for a period of twenty -
nine years, until his death on February 26, 1878.
To Secchi is due the credit of the first spectro-
scopic survey of the heavens. He reviewed the
spectra of 4000 stars, and classified them into
four distinct groups, which are recognised to
this day. The first type embraces over half
of 'those which Secchi examined. This type is
represented by Sirius, Vega, Altair, and other
bluish -white stars, and is characterised by the
intensity of the hydrogen lines. The second
type embraces the yellow stars, such as Capella,
Arcturus, Aldebaran, Pollux, and the Sun itself,
and is known as the Solar type. The spectra
of these stars closely resemble that of the Sun,
and are distinguished by innumerable lines.

THE LIGHT OF THE STARS. 171

Secchi's third type, or red stars, represented by
Betelgeux, Antares, and others, are characterised
by strong absorption bands, and the spectra have
been described as " fluted." The third- type stars
are comparatively scarce compared with the first
and second, and the fourth is even less numerous.
The fourth -type stars are also red with broad
absorption lines. To Secchi's four types a fifth
was added in 1867 by Wolf and Rayet of Paris
Observatory — namely, the gaseous stars. Secchi
aimed at a comprehensive survey of the stellar
spectra, and he accomplished much valuable
work. He did not devote his time to analys-
ing individual stars. This branch of study —
analysis of spectra and the determination of the
elements in the stars — was undertaken by his
contemporary, William Huggins, one of the
greatest astronomers whom England has ever
produced.

Born in London in 1824, William Huggins
commenced his astronomical researches at the
age of twenty -eight. In 1856 he erected, at
Tulse Hill, London, an observatory which he
equipped at great expense. He commenced
observations on the usual astronomical lines,
taking times of transits and making drawings
of the surfaces of the planets. But he soon
tired of the routine of ordinary astronomical

172 A CENTURY'S PROGRESS IN ASTRONOMY.

work, and on the publication of Kirchhoff's
explanation of the Fraunhofer lines in the solar
spectrum, he commenced to investigate the
spectra of the stars. Having constructed a suit-
able spectroscope, he commenced observations in
1862 in conjunction with his friend, William
Allen Miller, Professor of Chemistry in London.
He exhaustively investigated the two red stars,
Betelgeux and Aldebaran, ascertaining the exist-
ence in the former star of sodium, iron, calcium,
magnesium, and bismuth ; and in the latter star
the same elements, with the addition of tellurium,
antimony, and mercury.

In 1863 Huggins made an attempt to photo-
graph the spectra of the stars, and, indeed,
obtained prints of Sirius and Capella, but no
lines were visible in them. In 1874 Draper of
New York obtained a photograph of the spectrum
of Vega, showing four lines. Two years later
Huggins again attacked the problem, and secured
a photograph of the spectrum of Vega, showing
seven strong lines. In 1879 he was enabled to
communicate satisfactory results of his work to
the Royal Society, and since then he has secured
many admirable representations. ID 1899 the
monumental work, ' An Atlas of Representative
Stellar Spectra/ the joint work of Sir William
and Lady Huggins, was published.

THE LIGHT OF THE STARS. 1*73

In 1874 the German Government established
at Potsdam the Astrophysical Observatory, for
the spectroscopic study of the Sun and stars.
A position on the staff was given to Hermann
Carl Vogel, whose researches in astronomical
spectroscopy rank with those of Secchi and
Huggins. Born in Leipzig in 1842, he was from
1865 to 1869 employed in the Leipzig Observa-
tory. Called to Bothkamp as director in 1870,
he resigned his post in 1874 to accept a position
on the staff at Potsdam Observatory. In 1882
he became director of that Institution, which
position he still retains.

In 1874 Vogel revised Secchi's classification of
stellar spectra, and in 1895 he further improved
on it. His classification improves rather than
supersedes the previous work of Secchi ; never-
theless, he approached the question from a
different standpoint. Vogel concluded in 1874
that a rational scheme of stellar classification
"can only be arrived at by proceeding from the
standpoint that the phrase of development of the
particular body is, in general, mirrored in its
spectrum." Vogel divides Secchi's first type into
three classes. In the first type, designated la, —
represented by Sirius and Vega, — the metallic
lines are " very faint and fine," and the hydrogen
lines conspicuous. In 16 no hydrogen lines are

174 A CENTURY'S PROGRESS IN ASTRONOMY.

visible, while in Ic the hydrogen lines are bright.
This class includes the gaseous stars. In 1895,
after the recognition of helium in the stars by
his assistant, Scheiner, Vogel separated the stars
of class 16 from the first type altogether. These
stars are sometimes designated as " Type O," and
sometimes as helium stars and Orion stars, as
the majority of the stars in Orion are of that
type. The solar type is divided into two classes,
Ila being represented by the Sun, Capella, and
other well-known stars, while 116 includes the
Wolf- Rayet stars. Secchi's third and fourth
types are both classified by Vogel as of the
third type. These red stars were specially
studied from 1878 to 1884 by Duner at Lund.
His results were published in a descriptive cata-
logue which appeared at Stockholm in 1884.
His researches related to the spectra of 352
stars, 297 of Secchi's third type and 55 of his
fourth. Duner is perhaps the greatest authority
on stars with banded spectra.

Vogel's classification of spectra is generally
adopted by astronomers, although others have
been proposed by Lockyer and by Edward Charles
Pickering (born 1846), director of the Harvard
Observatory. Lockyer's classification was de-
signed to fit in with his " meteoritic hypothesis,"
discussed in the chapter on Celestial Evolution.

THE LIGHT OF THE STABS. 175

The stars were divided by Lockyer into seven
groups, according to his views of their tempera-
ture, rising through gaseous stars, red stars of
Secchi's third type, and a division of solar stars
to the Sirian type, and falling through a second
division of the solar type to red stars of Secchi's
fourth type.

The first spectroscopic star - catalogue was
published in 1883 by Vogel, assisted by Gustav
Mutter (born 1851), a son-in-law of Sporer. The
catalogue contained details of 4051 stars to the
seventh magnitude, and more than half of these
proved to be of Secchi's first type. Vogel's work
was completed in different latitudes by Duner
at Upsala, and by Nicolaus Thege von Konkoly
(born 1842) at O'Gyalla in Hungary.

The famous * Draper Catalogue ' ranks as the
greatest catalogue of stellar spectra. It was
undertaken at Harvard Observatory by E. C.
Pickering, in the form of a memorial to Henry
Draper, the successful spectroscopist. Com-
menced in 1886, and published in 1890, it con-
tains photographs of the spectra of no fewer
than 10,351 stars, down to the eighth magnitude.
Pickering subdivided Secchi's types into various
classes, the first or Sirian into four classes, the
second into eight, while the third and fourth
types each constitute a separate class. Pickering

176 A CENTURY'S PROGRESS IN ASTRONOMY.

designated his classes by the capital letters of
the alphabet.

Much useful work has been done also in the
analysis of the various spectra. Julius Scheiner,
now " chief observer" at Potsdam Astrophysical
Observatory, has, since 1890, done much valuable
work in this direction. Special attention was
devoted to the spectrum of Capella, 490 lines in
the spectrum of which were measured by Scheiner.
In his own words, " he believes a complete proof
of the absolute agreement between its spectrum
and that of the Sun to be thereby furnished."
Other stars of the Sirian and solar classes were
exhaustively studied by Scheiner.

The study of the exact brilliance of the stars
was a branch of research long neglected, yet it
is of much importance in astronomy, for it is only
through exact measurement of stellar brilliance
that stellar variation can be detected. Herschel
commenced the study, which was continued by
his son at the Cape, but it is only within the
last twenty years that stellar photometry has
become a recognised branch of astronomy ; and
the credit of this is due to the energy and zeal
of the great American observer, Edward Charles
Pickering.

Born in Boston in 1846, Edward Charles
Pickering was appointed in 1865 Instructor of

THE LIGHT OF THE STARS. 177

mathematics in the Lawrence Scientific School
at Harvard, after a distinguished university
career. In 1876 he succeeded Winlock as
director of the Harvard Observatory, and in the
following year he commenced his photometric
studies. He invented an instrument named the
meridian photometer, with the aid of which he
succeeded in determining, in the years 1879 to
1882, the exact brilliance of 4260 stars to the
sixth magnitude between the north celestial pole
and thirty degrees of south declination. At a
later date he devised a larger photometer, with
which he made over one million observations.
Pickering next extended his survey to the
southern hemisphere, erecting the photometer on
the slope of the Andes, where the Harvard
auxiliary station at Arequipa is now located,
and where 8000 determinations of stellar brilli-
ance were made. Meanwhile Pritchard, at Ox-
ford, published in 1885 his * Uranometria Nova
Oxoniensis,' with photometric determinations of
the brilliance of 2784 stars from the pole to
ten degrees of south declination. Both of
these catalogues were epoch-making works, and
testify to the enthusiasm and perseverance of
the astronomers who designed them.

The study of stellar photometry glides into
that of stellar variation. At the beginning of

M

178 A CENTURY'S PROGRESS IN ASTRONOMY.

the nineteenth century the number of known
variable stars was very small, as a glance at the
list given in Brewster's edition of Ferguson's
Astronomy (1811) will show. Some remarkable
investigations were due to the English astron-
omer, John Goodricke (1764-1786), who redis-
covered the variability of the star Algol, and
accurately determined its period in 1782. Good-
ricke suggested that the regular variations in
the light of Algol were due to the partial
eclipse of its light by a dark satellite, a hypo-
thesis now fully confirmed. Two years later, in
1784, Goodricke discovered other two variables,
8 Cephei and y8 Lyreo. He died in 1786 at the
age of twenty-one, and thus variable-star astron-
omy was deprived of its founder.

The foundation of variable-star astronomy as
an exact branch of the science is due to Arge-
lander. In the years 1837-1845, while residing
at Bonn during the erection of the observatory,
of which he had been made director, he erected
a temporary observatory, and there he carefully
determined the magnitudes of all stars visible in
Central Europe. From this he was led to the
discussion of stellar variation, to which subject
he continued to give much attention. He was
the first to describe a method of observing
variable stars scientifically and accurately, — a

THE LIGHT OF THE STABS. 179

method consisting in estimating in " steps " or
" grades" the difference in brilliance between the
variable, or suspected variable, and other stars
which are selected for comparison, and which
are of various degrees of brilliance, so that they
may be available for comparison with the vari-
able throughout its fluctuations. Argelander's
" steps " are tenths of a magnitude, and Gore
describes the method of observation as follows :
"If we call a and b the comparison stars, and v
the variable, a being brighter than b, and if v
is judged to be midway in brightness between a
and 6, we write a5v5b. If v is slightly nearer to
6, we write a§v4b. We may also write a3v7b,
or a7v3b, the sum of the steps being always
ten."

This method, described in 1844, led to many
discoveries at Bonn in the following twenty
years by Argelander and his assistants Schmidt
and Schonfeld. At this time Eduard Heis
(1806-1877), at Minister, who also ranks as one
of the founders of meteoric astronomy, while
engaged on the construction of his great atlas,
attentively determined the change of magnitude
of stars visible to the naked eye ; and by means
of the naked eye, the opera-glass, and a small
telescope, he amassed a large number of observa-
tions. At the same time two English observers,

180 A CENTURY'S PROGRESS IN ASTRONOMY.

Hind and Pogson, were making remarkable dis-
coveries which greatly increased the number of
known variables. Among Hind's discoveries
were S Cancri of the Algol type ; while
Schmidt discovered another of the same class,
8 Librae, and also the famous £ Geminorum.
While director of the Observatory of Mannheim,
an institution equipped with very antiquated
instruments, Schonfeld devoted himself to the
study of variable stars, and increased the num-
ber of known variables considerably. In the
southern hemisphere Gould, in South America,
did for the observation of variable stars what
Argelander did in the northern.

In 1874 a very important, although not so
obvious, service to variable-star astronomy was
rendered by the Danish observer, Hans Carl
Fredrik Christian Schjellerup (1827-1887). He
translated from Arabic into French the works
of the Persian astronomer of a thousand years
ago, Al-Sufi, and thus rendered his observations
available to modern astronomers. Al-Sufi was
a most accurate observer, and, by comparing
his catalogue with those of modern observers,
it can be found whether stars have changed
in brilliance during the past thousand years.

The study of variable stars has been pursued
in recent years by many astronomers, both by

THE LIGHT OF THE STABS. 181

means of photography and by the visual method.
The most important names in the visual dis-
covery of variables are Gustav Miiller and Paul
Friedrich Ferdinand Kempf(bom 1856) of Pots-
dam ; Alexander William Roberts of Lovedale,
South Africa ; Seth Carlo Chandler of Boston ;
Nils Christopher Duner at Upsala ; and John
Ellard Gore (born 1845) in Dublin.

The researches of J. E. Gore are a brilliant
example of how much may be done for astronomy
by means of very moderate instruments. Born
in 1845 at Athlone, in Connaught, he went to
India in 1868 as engineer on the Sirhind Canal
in the Punjab. While in India he erected his
small telescopes on brick pillars, and took ob-
servations, many of them of stellar brilliance.
In 1879 he returned to Ireland, and since then
has devoted himself to astronomy with zeal and
enthusiasm. His discoveries and investigations
of variables have been made by means of the
binocular. On December 13, 1885, he discovered
a remarkable star in Orion, which was at first
considered to be temporary, and called " Nova
Orionis," but was afterwards found to be a long-
period variable star.

Recently photography has come much to the
front in the discovery of variable stars. Picker-
ing at Harvard, and Wolf at Heidelberg, have

182 A CENTURY'S PROGRESS IN ASTRONOMY.

particularly distinguished themselves in this
branch, and the number of known variables is
now very large, as every year brings fresh dis-
coveries, mostly by aid of photography. Many
of these newly-discovered variables are in star-
clusters and nebulae.

Pickering proposed in 1880 the following
classification of variable stars, which has been
adopted all over the scientific world : Class
I., temporary star ; Class II., stars undergoing
in several months large variations, such as
Mira Ceti and U Orionis ; Class III., irregular
variables, such as Betelgeux and a Hercu-
lis; Class IV., short -period variables, such as
S Cephei, £ Geminorum, and ft Lyrse ; Class V.,
11 Algol variables," which undergo variations last-
ing but a few hours. It is doubtful whether
new stars should be included in a classification
of variables, although in one case, at least, a
new star was found to be a long-period variable.
To these a sixth class may now be added.
This class, the detection of which is mainly due
to the profound investigations of Gore, is com-
posed of what have been termed "secular vari-
ables," which undergo slow fluctuations in
periods of many years, and sometimes of cen-
turies. This Class includes 8 Ursse Majoris,
Al - Fard, X Draconis, 6 Serpentis, e Pegasi,

THE LIGHT OF THE STABS. 183

83 Ursae Majoris, £ Piscis Australia, fi Leonis,
a Ophiuchi, 77 Crateris, and others. The secular
variations of some of these stars have been
detected by Gore himself during the past thirty
years, while in other cases he has detected them
by comparison of the most important star-cata-
logues, from Hipparchus and Al-Sufi down to
our own time. In some cases the star in ques-
tion seems to be slowly gaining in brilliance,
in others slowly diminishing.

Thanks to the application of the spectroscope,
much is now known of the cause of the light
changes in variable stars. Goodricke's theory of
the variations of Algol was theoretically con-
firmed by the researches of E. C. Pickering in
1880. In 1889 Vogel proved beyond a doubt
that the variation in the light of Algol is due to
the partial eclipse of its light by a dark satellite.
It was obvious to Vogel that, as both Algol
and its companion are in revolution round their
common centre of gravity, the motion of Algol
in the line of sight might be detected by the
spectroscopic method of observation. Vogel
found that before each eclipse Algol was retreat-
ing from our system, while on recovering it gave
signs of rapid approach, proving conclusively that
both the star and its dark satellite were in
revolution round their centre of gravity, — Algol

184 A CENTURY'S PROGRESS IN ASTRONOMY.

suffering partial eclipse only because the plane
of the orbit lies in our line of sight. Algol,
therefore, is not inherently a variable star, but
merely a binary. Following up his researches,
Vogel, assuming that the bright and dark stars
are of equal density, arrived at the conclusion
that Algol is a globe about one and a half
million miles in diameter, the satellite equalling
the size of the Sun, and the centres of the stars
being separated by about 3,230,000 miles. Thus,
star- variables of the Algol type are not variable
in the true sense of the word. Even the most
irregular of the Algol variables have been ex-
plained. Perhaps the most irregular was Y
Cygni, discovered by Chandler in 1886. It was
soon found, however, that the variations recurred
with great irregularity : in less than two years
the phases differed by as much as seven hours
from the predicted times. At length the subject
was taken up by Dune'r at Upsala. A series of
observations made with the 14-inch refractor at
Upsala in 1891 and 1892 convinced him in the
latter year that two eclipses take place in the
course of one revolution : one star occults the
other. Duner showed that the intervals between
minima were thus — 1 day 9 hours; 1 day 15
hours; 1 day 9 hours, and so on. Thus, the
first, third, fifth, and seventh sets of minima

THE LIGHT OF THE STAKS. 185

obeyed a different law from the second, fourth,
sixth, and eighth. Duner proved that two stars
revolve round their centre of gravity in less
than three days, alternately occulting each other,
while the ellipticity of the orbit explains the
irregularity of the light changes. In April 1900
Duner gave his final conclusions as follows :
" The variable star Y Cygni consists of two stars
of equal size and equal brightness, which move
about their common centre of gravity in an
elliptical orbit, whose major axis is eight times
the radius of the stars/' He also stated the
exact period of revolution and the eccentricity
of the orbit.

In the case of the short - period variables,
such as ft Lyrae, 8 Cephei, £ Geminorum, and
T) Aquilae, the variations do not seem to be
due to eclipse. It was discovered by Professor
Pickering that ft Lyrse is a spectroscopic
binary, but Vogel and Keeler showed that
the supposed orbit is incompatible with the
eclipse theory. Vogel says : " I am convinced
that ft Lyrse represents a binary or multiple
system, the fundamental revolutions of which in
12 days 22 hours in some way control the light
change." The eclipse theory, however, is still
maintained by Belopolsky, who has framed a
hypothesis according to which the chief minimum

186 A CENTURY'S PROGRESS IN ASTRONOMY.

of the star's light corresponds with the obscura-
tion of the lesser star, the lesser minimum with
that of the primary, implying that the primary
is much less luminous in proportion to its light
than its satellite, — a state of affairs which Miss
Clerke concludes to be improbable.

The variable stars, 8 Cephei and T? Aquilae,
were both found in 1894 by Belopolsky to be
binaries ; but as the times of minimum light
do not correspond with those of eclipses in
the hypothetical orbits, he concludes that the
variations cannot be explained on the eclipsing
satellite theory. Miss Clerke is inclined to the
theory that the increase of luminosity in short-
period variables is due to tidal action, so that
while the revolutions of the stars control their
variability, they are inherently unstable in light.
A large number of these stars are known, and
it is a remarkable fact that the majority of these
variables lie on or near the Galaxy, so that their
variations have probably something to do with
their vicinity.

We now come to the long-period variables of
which Mira Ceti, x Cygni, and U Orionis are
examples. Although varying in regular periods,
generally of about a year, they are subject to
remarkable irregularities, so that an exact period
cannot be assigned even to Mira Ceti, of which

THE LIGHT OF THE STABS. 187

the maxima are at times retarded and at others
accelerated with no apparent law. The spectro-
scopic investigations of Campbell in 1898 have
shown that Mira Ceti is a solitary star, while
bright lines of hydrogen appear in its spectrum
at maximum, showing that the variations are
due to periodical conflagrations in its atmo-
spheres. In many other long - period variables
bright lines have been observed.

A remarkable fact regarding these stars is the
amount of their light change. Mira Ceti, for
instance, varies from the first to the ninth
magnitude, and U Orionis from the sixth to
the twelfth. As M. Flammarion says, "the
longer the period the greater the variation."
Another remarkable fact is that their light
curves show a curious resemblance to the curves
of the solar spots, only on a vastly greater scale,
which indicates that, relatively, these long-period
variables are much older than our Sun, the small
variations in the light of which are imperceptible.
"Here, if anywhere," says Miss Clerke, "will be
found the secret of stellar variability."

To the irregular variables no period can be
assigned. Betelgeux, in Orion, the variation of
which was noted by Sir John Herschel in 1840,
is a typically irregular variable. But the most
extraordinary of all variables is 77 Argus, in

188 A CENTURY'S PROGRESS IN ASTRONOMY.

the southern hemisphere, which is probably a
connecting link between variable and temporary
stars. The traveller Burchell, from 1811 to 1815,
observed the star as of the second magnitude,
but in 1827 he noted it to be of the first
magnitude. In the following year it fell to the
second magnitude. In 1834 Sir John Herschel
noted the star to be between the first and second
magnitude, and in 1838 it rose to the first, being
equal to a Centauri. After a decline, it be-
came in 1843 equal to Canopus, and not much
inferior to Sirius. Then it began to fade, and
in 1868 it was only of the sixth magnitude. In
1899 Innes estimated it as 771. Eudolf Wolf
suggested a period of 46 years, and Loomis
67 years ; but astronomers generally agree with
Schonfeld that the star has no regular period.

The first temporary star of the nineteenth
century was discovered by Hind, in London,
April 28, 1848. It was of the fifth magnitude
at maximum, and soon after began to fade,
falling to the tenth magnitude. In 1860 a new
star appeared in the cluster Messier 80 in Scorpio,
and was discovered by Auwers at Konigsberg.
It reached only the seventh magnitude.

On the night of May 12, 1866, a new star
of the second magnitude blazed out in the con-
stellation Corona Borealis. It was first observed

THE LIGHT OF THE STABS. 189

at Tuam, in Ireland, by the Irish astronomer,
John Birmingham. Four hours earlier Schmidt
had been observing that part of the heavens,
and it was not then visible. Birmingham at
once communicated the discovery to Huggins,
at Tulse Hill, who had commenced his spectro-
scopic observations. On May 16 Huggins ob-
served its spectrum. In the words of Miss
Clerke, " The star showed what was described
as a double spectrum. To the dusky flutings
of Secchi's third type, four brilliant rays were
added. The chief of these agreed in position
with lines of hydrogen ; so that the immediate
cause of the outburst was plainly perceived to
have been the eruption, or ignition, of vast
masses of that subtle kind of matter." Nine
days after the appearance of the new star it
was invisible to the naked eye, and afterwards
fell to the tenth magnitude. In 1856 Schonfeld
had observed it at Bonn as a telescopic star,
so that it was not a "new star" in the true
sense of the word.

The next temporary star observed was dis-
covered by Schmidt, at Athens, November 24,
1876. It was of the third magnitude, situated
in the constellation Cygnus. On December 2
its spectrum was examined at Paris by Alfred
Cornu (1841-1902), and some days later at

190 A CENTURY'S PROGRESS IN ASTRONOMY.

Potsdam by Vogel and Lohse. It was closely
similar to that of the new star of 1866, bright
lines of hydrogen and other elements standing
out in front of an "absorption" spectrum. By
the end of 1876 the star was of the seventh
magnitude. On September 2, 1877, Nova Cygni
was observed at Dunecht, and its spectrum was
found to have been transformed into that of a
planetary nebula. Three years later, however,
the ordinary stellar spectrum reappeared.

A new star appeared in the centre of the
great nebula in Andromeda in August 1885.
The first announcement of the discovery was by
Karl Ernst Albrecht Hartwig (born 1851), who
observed the new star on August 31 ; but it
had been previously seen by several other ob-
servers. On September 1 it was of the seventh
magnitude, and by March of the following year
had fallen to the sixteenth. Observed by Vogel,
Young, and Hasselberg, the new star gave a
continuous spectrum, but Huggins and Copeland
succeeded in discerning bright lines. Hall, at
Washington, undertook a series of measures to
detect the parallax of Nova Andromedse, but
his efforts were unsuccessful.

The discovery of the next temporary star was
announced February 1, 1892, by the Rev. Thomas

THE LIGHT OF THE STARS. 191

D. Anderson, a Scottish amateur astronomer, in
a post-card to the Astronomer-Royal of Scotland.
The star was situated in the constellation Auriga.
An examination of photographs, taken at Harvard
Observatory, showed that the new star had ap-
peared December 10, 1891, and had risen to
a maximum of the fourth magnitude ten days
later. On a photograph taken by Max Wolf
on December 8 the new star was not visible.
After Anderson's visual discovery, the spectrum
of the new star was studied by Copeland,
Huggins, Lockyer, Vogel, Campbell, and others.
Bright hydrogen lines were visible in the spec-
trum, which appeared to be actually double,
giving support to the theory that the outburst
was the result of a collision between two dark
bodies ; and this was confirmed by the measure-
ments of radial motion by the Potsdam astron-
omers.

After March 9, 1892, the new star steadily
faded, falling to the sixteenth magnitude on
April 26. But on August 17 Edward Singelton
Holden (born 1846), director of the Lick Observ-
atory, and his assistants, Schaeberle and Camp-
bell, found it of the tenth magnitude. On
August 19 Barnard found it transformed into a
planetary nebula : while various spectroscopic

192 A CENTURY'S PROGRESS IN ASTRONOMY.

observations of the revived Nova revealed the
nebular lines. By the end of 1894 the new
star had faded to the eleventh magnitude, and
early in 1901 was observed as a minute nebula.

After 1892 several new stars appeared, and
were detected on photographic plates by Mrs
Fleming (born 1857), of Harvard Observatory.
The first of these, in the southern constellation
Norma, was discovered in 1893 by its peculiar
spectrum on a Draper spectrographic plate taken
at Harvard. But the new star rose only to the
seventh magnitude. Other new stars were dis-
covered in Carina (Argo) in 1895, in Cen-
taurus in 1895, in Sagittarius in 1898, and in
Aquila in 1900. Nova Sagittarii was, at its
brightest, fully equal to Nova Aurigae, and was
plainly visible to the naked eye, but was never
observed visually.

A temporary star, appropriately designated
" the new star of the new century," blazed out
in Perseus on the night of February 21, 1901.
It was discovered independently by several ob-
servers : on February 21, by Borisiak, a student
at Kiev, in Russia ; on the following morning,
by Anderson in Edinburgh ; and on the next
evening, by Gore at Dublin, Nordvig in Den-
mark, Grimmler at Erlangen, and other ob-
servers. When first seen by Anderson, it was

THE LIGHT OF THE STARS. 193

equal to Algol, of the second magnitude. A
photograph by Williams at Brighton showed
that it must have been fainter than the twelfth
magnitude on February 20. On the evening
of February 23 the star was brighter than
Capella, and was then the brightest star in the
northern hemisphere. On February 25 it fell to
the first magnitude ; on March 1 to the second,
and on March 6 to the third. During the spring
and summer the light fluctuated considerably,
but in September and October faded to the
6*7 magnitude. In March 1902 it was of the
eighth magnitude, and in July 1903 of the
twelfth.

The spectrum of Nova Persei was found by
Pickering to be of the Orion type on February
22 and 23. On February 24 the spectrum had
become one of the bright and dark lines, and
the hydrogen lines indicated a velocity of 700
to 1000 miles a second. Measures of the sodium
and calcium lines, by Campbell and others, in-
dicated a velocity of only three miles a second,
so that the displacements of the hydrogen lines
may have been due to an outburst of hydrogen
in the star. The spectrum was carefully studied
during the spring and summer by Pickering,
Lockyer, Huggins, Vogel, and others. On June
25 Pickering reported that the spectrum was

N

194 A CENTURY'S PROGRESS IN ASTRONOMY.

slowly changing into that of a gaseous nebula.
In August and September 1901 the nebular
spectrum became more apparent.

In August 1901 Wolf at Heidelberg discovered
a faint trace of nebula near the nova. On Sep-
tember 20 this nebula was photographed by
George Ritchey at the Yerkes Observatory, and
was seen to be of a spiral form. This was con-
firmed by Perrine, who also found, from plates
taken in November, that the nebula was moving
at the rate of eleven minutes of arc a year.
This extraordinary velocity was exceedingly
puzzling to astronomers, and at length Kapteyn
suggested that the nebula shone only by re-
flected light from the new star, and that the
apparent motion was an illusion caused by the
flare of the explosion travelling out from the
nova.

On March 16, 1903, Herbert Hall Turner
(born 1861), Professor of Astronomy at Oxford,
discovered a new star of the seventh magnitude
in the constellation Gemini, from an examination
of photographic plates. Photographs taken at
Harvard showed that on March 1 it must have
been fainter than the twelfth magnitude, while
five days later it was of the fifth. In August
1903 Pickering found its spectrum nebular. In
August 1905 another small nova was found by

THE LIGHT OF THE STABS. 195

Mrs Fleming on the Harvard photographs, situ-
ated in Aquila.

Many theories have been advanced to account
for temporary stars. Flammarion has shown
that a body surrounded by a hydrogen atmo-
sphere, on grazing a dark body enveloped in
oxygen, would produce a tremendous explosion.
In 1892 Huggins suggested that the outburst
of Nova Aurigae was due to the near approach
of two bodies with large velocities, disturbances
of a tidal nature resulting and producing enor-
mous outbursts. Vogel suggested that the new
star was due to the encounter of a dark star
with a worn - out system of planets ; while
Lockyer believes all new stars to be due to
the collision of swarms of meteors. Perhaps
the most probable theory is that of Seeliger,
which attributes these outbursts to the move-
ment of a dark body through nebulous matter,
which is extensively diffused throughout space.
This theory explains the changes in the spectra
as well as the revivals of brightness which
characterised Nova Aurigse and the fluctations
of Nova Persei. In a paper read to the Royal
Society of Edinburgh in November 1904, the
German astronomer, Jacobus Halm, of the Royal
Observatory, Edinburgh, extended and developed
Seeliger's theory, showing also that the necessary

196 A CENTURY'S PROGRESS IN ASTRONOMY.

consequence of such an encounter as the theory
assumes is the formation of an atmosphere of
incandescent gases, followed by that of a re-
volving ring of nebulous matter. In the hands
of Halm, therefore, Seeliger's theory leads to
the nebular hypothesis as advanced by Laplace
and Herschel.

CHAPTER XL

STELLAR SYSTEMS AND NEBULJE.

THE study of double stars, commenced by
Herschel, was taken up after his death by
several of the foremost astronomers, and has
since been pursued by quite a number of ob-
servers and computers. Herschel's immediate
successor in the study of double stars was his
son, who ranks only second to his father as a
student of stellar systems. Born at Slough on
March 7, 1792, John Frederick William Herschel
passed his childhood " within the shadow of
the great telescope." Although his early life
was spent with his father and aunt, astronomy
does not appear to have taken up his attention
as a boy. Chemistry, however, always inter-
ested him, and, as his aunt recorded, even while
a child he was fond of making experiments.
He was educated at Hitcham, and afterwards
at Eton. He was delicate, however, so his
mother removed him from school, and he was

198 A CENTURY'S PROGRESS IN ASTRONOMY.

trained at Slough by Mr Rogers, a Scot-
tish mathematician. At the age of seventeen
Herschel entered the University of Cambridge,
and Caroline Herschel, who was exceedingly
proud of him, recorded in her memoirs that he
gained all the first prizes without exception.
He left the University in 1813.

John Herschel did not turn his attention to
astronomy until he had attained the age of
twenty-four. In a letter to a friend, September
10, 1816, he said, "I am going, under my
father's directions, to take up star-gazing." It
was only reverence for his father that made him
turn to astronomy, and he gave up the science
he loved most — chemistry. But his unselfishness
received its reward. In 1820 John Herschel
constructed his first reflector under his father's
guidance. Four years previously he had begun
to observe double stars, which had been for long
studied by his father, who discovered their
revolutions. These observations were continued
from 1821 to 1823 at the Observatory of Sir
James South (1786-1867). John Herschel and
South measured 380 of the elder Herschel's
double stars. These investigations gained for
Herschel and South the Lalande Prize of the
French Academy and the Gold Medal of the
Royal Astronomical Society.

STELLAR SYSTEMS AND NEBULA. 199

When his mother died Sir John Herschel
decided to sail to the Cape of Good Hope to
make an investigation of the stars of the
southern hemisphere, which until then had been
much neglected. He was offered a free passage
in a ship of war, but declined. In November
1833 he left England, taking with him his great
telescopes. In two months he arrived at Cape
Town, and erected his astronomical instruments
at Feldhausen, a short distance off. In October
1835 he informed his aunt that he had almost
completed his survey of the southern hemisphere.
During his "sweeps" of the heavens he dis-
covered 1202 double stars, and 1708 nebulae
and star-clusters. In 1838 he returned to Eng-
land, and devoted the remainder of his life to
the publication of his results, as well as to
other branches of science. He died at Colling-
wood, in Kent, on May 11, 1871, at the age of
seventy-nine.

John Herschel's favourite objects of study
were double stars, of which he discovered 3347
in the northern hemisphere, and 2102 in the
southern. He also computed several stellar
orbits ; but the first calculation of a stellar
orbit was made by the French astronomer Felix
Savary (1797-1841), who computed the orbit of
f Ursae Majoris, and found the period to be

200 A CENTURY'S PROGRESS IN ASTRONOMY.

about sixty years. Contemporary with John
Herschel was his great rival in double - star
astronomy, Friedrich Georg Wilhelm Struve.
Born at Altona in 1793, Struve took his degree
in 1811 at the Russian University of Dorpat.
In 1813 he became director of the Dorpat
Observatory, and was in 1839 promoted to
Pulkowa, as director of the great Observatory
there, remaining at its head until within three
years of his death, on November 23, 1864.
Struve's first recorded observation was on the
double star Castor. In 1819 he commenced to
measure the position-angles of double stars, of
which he published a catalogue of 795. In
1825 he commenced a review of the heavens
down to fifteen degrees south, and thus dis-
covered 2200 previously unknown objects. The
results were published in Struve's ' Mensurae
Merometricae,' which appeared in 1836, giving
the places, distances, colours, position - angles,
and relative brilliance of 3112 double and mul-
tiple stars.

Struve's successor in this branch of astronomy
was his son, Otto Wilhelm von Struve, born in
1819 at Dorpat, who became in 1837 assistant
to his father, and in 1861 succeeded him as
director of the Pulkowa Observatory. In 1890
he retired from this post, settling in Germany,

STELLAR SYSTEMS AND NEBULJS. 201

at Carlsruhe, where, on April 14, 1905, he died
in his eighty-sixth year. Otto Struve detected
500 double stars, among them y Andromedse,
discovered in 1842, and 8 Equulei, discovered
in 1852, within a period of between five and
eleven years.

Various other astronomers have devoted them-
selves to the observation of double stars, among
them Ercole DembowsJci (1815-1881), of Milan;
Karl Hermann Struve (born 1854), son of
Otto Struve; William Doberck (born 1845);
William J. Hussey (born 1864), now director of
the Detroit Observatory; Camille Flammarion;
N. C. Duner; G. V. Schiaparelli ; Thomas
Jefferson Jackson See (born 1866). But the
greatest living discoverer is Sherburne Wesley
Burnham (born 1838), now employed at the
Yerkes Observatory, in Wisconsin. Born in
1838 at Thetford, Vermont, he commenced his
career as a shorthand reporter, studying astron-
omy in his leisure hours. With a small 6-inch
refractor, mounted in a home-made observ-
atory, Burnham commenced in 1871 his dis-
coveries of double stars, which soon attracted
the attention of noted astronomers, who per-
mitted him to use larger telescopes, with which
he continued his researches. His first official
appointment was in 1888, when he became

202 A CENTURY'S PROGRESS IN ASTRONOMY.

chief assistant at the Lick Observatory, which
position he resigned in 1892. Some years later
he became astronomer in the Yerkes Observa-
tory. Altogether he has discovered 1308 double
stars, with telescopes ranging from a 6 - inch
refractor to the gigantic 40 -inch of the Yerkes
Observatory.

The computation of double-star orbits has been
undertaken by various astronomers, among them
Madler, Klinkerfues, Duner, Flammarion, Seeliger,
See, Gore, Burnham, Robert Grant Aitken (born
1864) of the Lick Observatory, and Giovanni
Celoria (born 1842), who was, from 1866 to
1900, assistant in the Brera Observatory of
Milan, and since 1900 director of that institu-
tion. On June 9, 1890, Gore presented to the
Royal Irish Academy a catalogue of computed
binaries containing reference to fifty-nine stars.

In 1844 Bessel discovered a remarkable irreg-
ularity in the proper motion of Sirius. He
ascribed this to the gravitational influence of
some obscure body, probably a large satellite.
In 1857 Peters calculated an orbit for the
supposed satellite with a period of 50 years.
In 1861 an orbit was computed by Truman
Henry Safford (1836-1901), which indicated the
position of the satellite. Close to this position
it was accidentally discovered by Alvan Clark

STELLAR SYSTEMS AND NEBULAE. 203

(1832-1897), the famous American optician.
The period of the star seems to be about 50
years. In 1844 Bessel noticed irregularities in
the proper motion of Procyon, and put forward
the idea of a disturbing satellite, as in the case
of Sirius. This was confirmed by Madler, and
in 1874 an orbit was computed by Auwers, who
found a period of 40 years. In 1896 the
satellite was found by Schaeberle with the
36-inch refractor of the Lick Observatory. A
period of 40 years was found by See, in
agreement with the hypothetical orbit.

In putting forward these theories as to in-
visible stellar satellites, Bessel remarked that
"light is no real property of mass," and that
the existence of countless visible stars is nothing
against the existence of countless invisible and
dark ones. In this he laid the foundation of
the branch of science termed by Madler the
" Astronomy of the invisible." In recent years
the astronomy of the invisible has become
a recognised branch of astronomical research,
through the application and interpretation of
Doppler's principle in spectroscopic observations.
In the course of photographing the stellar
spectra for the Draper Catalogue, E. C. Pick-
ering photographed the spectrum of Mizar
(£ Ursae Majoris) in 1887 and again in 1889. On

204 A CENTURY'S PROGRESS IN ASTRONOMY.

some of these photographs the line K was seen
double, while on others it was seen under its
normal aspect. This doubling of the lines
indicated that the star which we see as single
is in reality composed of two bodies in revolu-
tion round their centre of gravity, so close
together that even the largest telescopes cannot
divide them. Pickering assigned a period of
104 days, but in 1901 Vogel diminished this
to 20 days. In the same year the star
ft Aurigae was similarly found to be double ;
and in 1890 Vogel, from photographs taken
at Potsdam, independently inaugurated the
discovery of spectroscopic binaries. In the
spectrum of Spica he discovered the spectral
lines to be, not doubled, but periodically dis-
placed, indicating the existence of a dark or
nearly dark companion, both stars revolving
round their centre of gravity. Spica was seen
to belong to the same class as Algol, only that
in the case of Algol the plane of the satellite's
orbit passes through the Earth and eclipses the
star, while in the case of Spica the orbit is
inclined, and the star is constant in light.

The line of research commenced by Vogel and
Pickering was soon followed up by these investi-
gators, as well as by Belopolsky at Pulkowa,
Campbell at the Lick Observatory, Slipher at

STELLAR SYSTEMS AND NEBULAE. 205

the Lowell Observatory, and by Edwin Brant
Frost (born 1866), now director of the Yerkes
Observatory, and his assistant, Walter Adams.
In 1894 Belopolsky discovered the duplicity of
several variable stars, and in 1896 that of Castor,
in Gemini. Late in 1896 Campbell undertook a
systematic investigation of radial motions, and
has since discovered about sixty spectroscopic
binaries, — among them, in 1899, the Pole Star,
and in 1900 Capella. The latter discovery was
made independently by Hugh Frank Newall
(born 1857) at Cambridge, in England. It was
found by Campbell that the revolution of the
stars round their centre of gravity is performed
in 104 days ; and it soon became apparent
that, owing to the large size of the orbit, the
duplicity of Capella might be observed tele-
scopically. At Greenwich the star was seen
to be elongated, but at the Lick Observatory
it was seen persistently single.

Campbell finds that of 285 stars observed by
him, more than one in nine is a spectroscopic
binary. He concludes that at least one star in
five or six will be found to be spectroscopically
double, and considers that " the proven existence
of so large a number of stellar systems, differing
so widely in structure from the Solar System,
gives rise to a suspicion at least that our

206 A CENTURY'S PROGRESS IN ASTRONOMY.

system is not of the prevailing type of stellar
systems."

The study of triple and multiple stars is of
deep interest, but the orbits of these objects
cannot be said to be fully investigated by any
means. The first application of the problem of
three bodies to stellar astronomy was made by
Seeliger in 1889. His researches, relating to
the famous star, £ Cancri, disclosed the exist-
ence of three stars revolving round a dark body,
apparently the most massive in the system.
The system of £ Cancri, at least, seems to be
modelled on the Ptolemaic design.

In the study of star-clusters and nebulae, as
in the investigation of double stars, Herschel's
successor was his son. His observations, both in
England and at the Cape of Good Hope, resulted
in a large number of new discoveries, and the
results of his studies in this direction were
published in 1864 in his catalogue of all known
clusters and nebulae, amounting to 5079. This
catalogue was enlarged and revised in 1888 by
John Louis Emil Dreyer (born 1852), a Danish
astronomer, but director of the Observatory at
Armagh, in Ireland ; and the same observer
published from 1888 to 1894 a supplementary
list, bringing the number of known clusters and
nebulae to about 10,000.

STELLAR SYSTEMS AND NEBULJE. 207

In the early part of his career, John Herschel
held firmly to the views of his father of the
difference between star-clusters and nebulae, con-
sidering the latter to be composed of "shining
fluid." But he fell off from this view with the
resolution into stars of many irresolvable nebulae.
In 1845 William Parsons, third Earl of Rosse
(1800-1867), erected at Birr Castle, in Ireland,
his great 6 -foot reflector, which still surpasses
all other telescopes in point of size. With this
instrument Lord Rosse believed himself to have
resolved the Crab nebula in Taurus and the
Nebula in Orion, which was also said to have
been resolved by Bond with the 15 -inch refractor
at Harvard; and in 1854 Olmsted declared the
"resolution" of these nebulae to be the signal
for the renunciation of Herschel's nebular theory.
Most astronomers fell in with the view that all
the nebulae were distant clusters, which would
eventually be resolved into stars, although it is
only right to state that the Scottish astronomer,
John Pringle Nichol (1804-1859), and some other
investigators, held to the theory of Herschel.

The solution of the great problem was in 1864,
when on August 29 of that year Huggins turned
his spectroscope on a bright planetary nebula in
Draco. To his amazement the spectrum was
one of bright lines, proving conclusively that the

208 A CENTURY'S PROGRESS IN ASTRONOMY.

nebula was not a star-cluster, but a mass of glow-
ing gas, — hydrogen, and some other unknown
substance, now named "nebulium." By 1868
Huggins had observed the spectra of seventy
nebulse. Of these one - third proved to be
gaseous, among them the great Orion nebula
which Lord Rosse was believed to have resolved
into stars. In the spectrum of the latter, the
" chief nebular line " was at first ascribed by
Huggins to nitrogen, but this was a mistake.
Later, it was believed by Lockyer to coincide
with the fluting of magnesium, but this was
disproved by Huggins in 1889-90, and by
Keeler in 1890-91. The great nebula in
Andromeda and the great spiral in Canes
Venatici were found by Huggins to display a
continuous spectrum, and a similar discovery
was made in regard to the cluster M 13 in
Hercules, and other star-clusters. In the case
of the nebulae, it is not believed that the con-
tinuous spectrum is due to the existence of sun-
like bodies, as a gas under pressure would give
a continuous spectrum.

The Orion nebula has been more thoroughly
studied than any other object of its class. The
application of photography to spectroscopy has
done much to further the study of the lines in
the nebular spectrum. In 1886 Copeland de-

STELLAR SYSTEMS AND NEBULA. 209

tected in the spectrum of the Orion nebula the
yellow ray of helium. On February 13, 1890,
Scheiner announced an important discovery,
namely, the possession by both the nebula and
the stars in Orion — with the exception of Betel-
geux — of a line, which appeared bright in the
nebular spectra and dark in the stellar. This
line was identified by Vogel, Lockyer, and others
with that of helium.

Nebular photography was inaugurated in 1880
by Draper, who obtained a remarkably good
representation of the Orion nebula in that year.
His work in this direction, cut short by his
death in 1882, was taken up by Janssen at
Meudon, and by Common in England, who
obtained, in 1883, several excellent photographs.
Later photographs have shown the Orion nebula
to be much more extended than visual observa-
tions would lead one to expect. A photograph
secured in 1890 by W. H. Pickering revealed
the nebulous matter in Orion in its true form,
that of a gigantic spiral, starting from near
Bella trix, sweeping past K Orionis and Rigel to
17, and joining with the great nebula surrounding
0; the entire constellation being thus shown to
be enwrapped in nebulous haze.

In 1885 nebular photography was commenced
by Isaac Roberts (1829 - 1904), the English

210 A CENTURY'S PROGRESS IN ASTRONOMY.

amateur astronomer, who secured admirable
representations of clusters and nebulae. He
published, in 1893 and 1900, two volumes of
collected photographs of clusters and nebulae.
This monumental work was thus referred to by
Dr William James Lockyer : " Dr Roberts has
not only nobly enriched astronomical science,
but has raised a monument to himself which
will last as long as astronomy has any interest
for mankind."

Perhaps the most remarkable revelation made
by photography in this branch of research has
been the discovery of the nebulae in the
Pleiades. In 1859 Tempel observed at Flor-
ence an elliptical nebula south of the star
Merope. On November 16, 1885, the brothers
Henry obtained at Paris a photograph of the
Pleiades, revealing the existence of a small
spiral nebula. This was confirmed by visual
observations, and particularly by the photo-
graphs of Roberts, which also showed the entire
cluster to be nebulous, and that " the nebulosity
extends in streamers and fleecy masses, till it
seems almost to fill the spaces between the stars,
and to extend far beyond them." In 1888 a
further advance was made by the brothers
Henry, who found seven stars to be strung on
a nebulous streak.

STELLAR SYSTEMS AND NEBULJS. 211

Since 1890 nebular photography has been
pursued by Max Wolf in Germany, and by
E. E. Barnard and J. E. Keeler in America.
Wolf's photographs of the constellation Cygnus
brought out the close connection between the
stars and the extensively diffused nebulosities
discovered by him. In 1901 Wolf discovered
a " nebelhaufen " or cluster of nebulse, and in
1902 published a catalogue of 1528 nebulae
round the pole of the Galaxy, showing them
to be systematically distributed. Keeler made
his memorable observations with the great
36 -inch reflecting telescope, which was con-
structed in England many years ago by
Common. It afterwards passed into the hands
of Mr Crossley of Halifax, who presented it to
the Lick Observatory. With this great instru-
ment Keeler commenced to take photographs
of the heavens. On one occasion he photo-
graphed a well-known nebula, and on develop-
ing the plate was surprised to find seven new
nebulae besides that which he had photographed.
On another occasion he exposed a plate to a
nebula in Pegasus. He was amazed to find
altogether twenty -one nebulae included in the
photograph. To give another instance, a plate
directed to the constellation Andromeda con-
tained no fewer than thirty-two nebulous objects.

212 A CENTURY'S PROGRESS IN ASTRONOMY.

This has given an enormous extension to our
knowledge of the nebulae. But even this is not
all. Keeler found on his plates numerous points
of light which seem to be also nebulae, either too
small or too remote to appear as such. Appar-
ently, however, they are not stars. Keeler's
work convinced him that, on a modest estimate,
there must be at least one hundred and twenty
thousand new nebulae within reach of the
Crossley reflector. Half of these, he announced,
were probably spiral. An idea of the vast
importance of Keeler's work may be gained if
we reflect that the observations of all the earlier
astronomers resulted in the discovery of six
thousand nebulae. The investigations of Keeler,
in all probability, were the means of adding
120,000 more.

Many observations have been made on nebulae,
for the purpose of ascertaining their proper
motions — but without success. Measurements
were made by D' Arrest in 1857 and by Burn-
ham in 1891, but none of these revealed any
motion of the nebulae across the line of sight.
Even the new spectroscopic method of deter-
mining motions in the line of sight, in the hands
of Huggins, failed in the case of the nebulae.
With the great Lick refractor at his disposal,
Keeler attacked the subject in 1890, and

STELLAR SYSTEMS AND NEBULAE. 213

measured the radial velocities of ten nebulae.
He found that the well-known planetary nebula
in Draco was moving towards the Solar System
at the rate of 40 miles a second; for the
Orion nebula he found a motion of recession of
11 miles a second; but probably this belongs
chiefly to the movement of the Solar System in
the opposite direction.

Unfortunately Keeler did not live to carry
on his investigations in nebular astronomy. His
early death brought to an abrupt end these
fruitful investigations. Appointed director of
the Lick Observatory in 1898, he died suddenly
at San Francisco on August 12, 1900, at the
early age of forty-two.

CHAPTEE XII.

STELLAR DISTRIBUTION AND THE STRUCTURE
OF THE UNIVERSE.

AFTER the death of Herschel there was little
done in the direction of furthering our knowledge
of stellar distribution, or the construction of the
heavens. Here, as elsewhere, Herschel's im-
mediate successor was his son, whose star-gauges,
both in England and in South Africa, were a
worthy sequel to those of his father ; but John
Herschel, in his books on astronomy, reproduced
his father's disc-theory, unaware that the elder
Herschel had himself abandoned it. The work of
the younger Herschel was entirely supplement-
ary to that of his father.

To Wilhelm Struve belongs the credit of
showing the disc-theory to be untenable, and of
demonstrating that Herschel had abandoned it.
This he was able to do after a perusal of
Herschel's papers, presented to him by John
Herschel. Having demonstrated this, he under-

STELLAR DISTRIBUTION. 215

took a series of investigations which resulted in
his famous theory of the Universe. This was
published in his work 'Etudes d'Astronomie
Stellaire,' which was published in 1847. His
researches were based on the star-catalogues of
Bessel, Piazzi, and others ; and dealing with
52,199 stars, he discussed the number of stars
in each zone of Right Ascension. He found,
in the words of Mr Gore, " that the numbers
increase from hour i to hour vi, where they
attain a maximum. They then diminish to a
minimum at hour xiii, and rise to another but
smaller maximum at hour xviii, again decreas-
ing to a second minimum at hour xxii. As the
hours vi and xviii are those crossed by the
Milky Way, the result is very significant."
He concluded the Galaxy to be produced by a
collection of irregularly-condensed clusters, the
stars condensed in parallel planes. Next, he
considered the Universe as perhaps infinitely
extended in the direction of the Galaxy, and
accordingly he put forward the idea that the
light from the fainter and more distant stars
was extinguished in its passage through the
ether of space, which he regarded as imper-
fectly transparent. The theory, as Struve pro-
pounded it, was disposed of by Sir John Herschel,
who remarked that we were not permitted to

216 A CENTURY'S PROGRESS IN ASTRONOMY.

believe that at one part of the sky our view
was limited by extinction, while at another a
clear view right through the Galaxy could be
had ; and by Robert Grant (1814-1892), director
of the Glasgow Observatory, who showed that,
were the theory true, the Galaxy should present
a uniform appearance throughout its course.
On the whole, Struve's theory was no improve-
ment on Herschel's ; for, as Encke pointed out,
Struve's theory was built on five assumptions,
all of which were questionable.

At the time of Struve's investigation Madler,
at Dorpat, was engaged in an attempt to solve
the question of the construction of the heavens
by quite another method, that of stellar proper
motion. He determined to investigate the sub-
ject of proper motion in order to discover the
central body of the Milky Way. If such a
centre existed, however, the motions near it
would be somewhat different from those in the
Solar System. In our Solar System the planets
nearest the Sun move swiftest, owing to the
strength of the force of gravitation. In the
Sidereal System, on the other hand, the move-
ments at the centre, as Madler pointed out,
would be slowest. As there would be no very
large preponderating body, the mutual attrac-
tions of the different stars would cause the bodies

STELLAR DISTRIBUTION. 217

at the boundaries of the Universe to move faster
than those at the centre, the central sun — the
object of Madler's search — being in a state of
rest relative to the Sidereal System. Madler
accordingly began to search the heavens for a
region of sluggish proper motions.

In the constellation Taurus, Madler noticed
that the proper motions of the stars were very
slow. The idea occurred to him that the bright
red star Aldebaran might be the central sun,
but its very large proper motion was obviously
against this inference. Star after star was now
subjected by Madler to the most careful scrutiny.
At length, after a laborious investigation, he
announced that the star which fulfilled the con-
ditions of a central body was Alcyone, the
brightest of the Pleiades, a group possessed of
no proper motion except that due to the sun's
drift in the opposite direction. In 1846 Madler
published his hypothesis in his elaborate work,
'The Central Sun/ He announced that his
observations had led him to the conclusion that
Alcyone occupied the centre of gravity of the
Sidereal System, and was the point round which
the stars of the Galaxy were all revolving. His
profound imagination, however, did not stop
here. This speculation led him to the sublime
thought that the centre of the Universe was

218 A CENTURY'S PROGRESS IN ASTRONOMY.

the Abode of the Creator. In 1847 Struve re-
jected Madler's theory as " much too hazardous,"
and this has been the general opinion of astron-
omers. Madler's theory is now regarded as
quite untenable.

Herschel's earlier idea that the nebulae were
external galaxies was long held by the majority
of astronomers, in preference to his later and
more advanced ideas. The supposed resolution
of the nebulae by Lord Bosse's telescope gave
support to this external galaxy theory. It was
clearly shown, however, by William Whewell
(1794-1866) in 1853, and by Herbert Spencer
(1820-1903) in 1858, that the systematic dis-
tribution of the nebulae in regard to the stars
precluded the possibility of their being external
galaxies. This was confirmed by the spectro-
scopic discovery of the gaseous nature of some
of the nebulae, and by the later researches of
R. A. Proctor. Not only did Proctor make fresh
discoveries, but it fell to him to clear away the
erroneous ideas regarding the construction of the
heavens, and to put the study on a new basis.
In 1870 Proctor plotted on a single chart all
the stars, to the number of 324,198, contained
in Argelander's ' Durchmusterung ' charts. This
work gave the death-blow to the " disc-theory."
In his own words, " In the very regions where

STELLAR DISTRIBUTION. 219

the Herschelian gauges showed the minutest
telescopic stars to be most crowded, my chart
of 324,198 stars shows the stars of the higher
orders (down to the eleventh magnitude) to be
so crowded, that by their mere aggregation
within the mass they show the Milky Way with
all its streams and clusterings. It is utterly im-
possible that excessively remote stars could seem
to be clustered exactly where relatively near
stars were richly spread."

Proctor showed also that in all probability the
stars composing the nebulous light of the Galaxy
are much smaller than the brighter stars, and
not at such a great distance as their faintness
would lead us to suppose, — a conclusion confirmed
by the work of Celoria. Proctor was not so for-
tunate in theorising as in direct investigation.
He thought that the Magellanic clouds were
probably external galaxies ; and further, he put
forward the idea that the Milky Way is a spiral,
the gaps and coal- sacks being due to loops in
the stream, but neither of these ideas has found
favour with astronomers. But the chief work
accomplished by Proctor was a revision of our
knowledge of the Universe, which he thus
describes : " Within one and the same region
coexist stars of many orders of real magnitude,
the greatest being thousands of times larger

220 A CENTURY'S PROGRESS IN ASTRONOMY.

than the least. All the nebulae hitherto dis-
covered, whether gaseous and stellar, irregular,
planetary, ring -formed, or elliptic, exist within
the limits of the Sidereal System."

Proctor's discovery of the excess of bright stars
on the Galaxy was confirmed by Jean CJiarles
Houzeau (1820-1888), director of the Brussels
Observatory. Some time later J. E. Gore care-
fully examined the positions of all the brighter
stars in the northern and southern hemisphere.
Following this, he made an enumeration of the
stars in the atlas of Heis and in the charts
constructed by Harding ; the outcome of the in-
vestigation being to show that stars of each
individual magnitude taken separately tend to
aggregate on the Galaxy, the aggregation being
noticed even in first - magnitude stars. Gore
further pointed out many cases of close con-
nection between the lucid stars and the galactic
light. A similar investigation was undertaken
by Schiaparelli in 1889. Schiaparelli, basing his
work on the catalogue of Gould and the photo-
metric measures of Pickering, constructed a series
of planispheres which demonstrated the crowd-
ing of the lucid stars towards the plane of the
Galaxy. These investigations were still further
continued by Simon Newcomb, who demonstrated
that " the darker regions of the Galaxy are only

STELLAR DISTRIBUTION. 221

slightly richer in stars visible to the naked eye
than other parts of the heavens, while the bright
areas are between 60 and 100 per cent richer
than the dark areas." The Dutch astronomer,
Charles Easton, finds a connection between the
distribution of ninth -magnitude stars and the
luminous and obscure spots in the Galaxy.

It was noticed by Gould, from observations
made at Cordova, that " a belt or stream of
bright stars appears to girdle the heavens
very nearly in a great circle which intersects
the Milky Way." According to Gould, the
belt includes Orion, Canis Major, Argo, Crux,
Centaurus, Lupus, and Scorpio in the southern
hemisphere, and Taurus, Perseus, Cassiopeia,
Cepheus, Cygnus, and Lyra in the northern.
This was interpreted by Celoria as indicating
the existence of two galactic rings, but Gould
considered the zone of bright stars to form with
the Sun a subordinate cluster of about five
hundred stars within the Galaxy.

Perhaps the most elaborate investigations on
the structure of the Universe have been those
of Kapteyn, commenced in 1891. In that year
he demonstrated that stars are bluer and
more easily photographed in the Galaxy than
elsewhere, a discovery independently made by
Gill at the Cape, and Pickering at Har-

222 A CENTURY'S PROGRESS IN ASTRONOMY.

vard. In 1893 Kapteyn announced his con-
clusions, derived from a novel method of
studying the distance of the stars from their
proper motions. In order to reach a definite
idea of the distances of the stars, he made use
of the component of the proper motion, meas-
ured at right angles to a great circle of the
sphere which passes- through a given star and
the apex of the solar motion. He found that
stars of the first spectral type have smaller
proper motions than those of the second, indi-
cating that stars of the second type are on the
average nearer to the Solar System than those
of the first, the near vicinity containing almost
exclusively second - type stars. Kapteyn con-
cluded that the group of second - type stars
formed one system, named the solar cluster,
which he considered to be roughly spherical in
shape. In 1902 he abandoned this idea, retain-
ing, however, his opinions as to the relative
distances of the different types. That the
second -type stars are nearer to the Sun than
the first is, he remarked in a letter to the
writer, incontrovertible.

In the investigation of the motions in, and
extent of, the Universe, the name of Simon
Newcomb stands out pre-eminently. Born in
1835 at Wallace, in Nova Scotia, he went to

STELLAR DISTRIBUTION. 223

the States in 1853. In 1862 he received an
appointment at Washington Observatory, and he
retained an official position until 1897. Through-
out his scientific career he has been specially
attracted by the question of the construction of
the heavens, which he fully discussed in his
book on 'The Stars' in 1901. Newcomb's in-
vestigations have shown that some of the stars
are not permanent members of the Sidereal
System, among them the swiftly -moving 1830
Groombridge. He has shown that the Stellar
Universe does not possess that form of stability
which is seen in the Solar System. Newcomb
considers the Universe to be limited in extent,
as opposed to the opinions of Struve and others,
who believed it to be infinite. He has brought
clearly before his readers a calculation, based on
the known law that there are three times as
many stars of any given magnitude as of that
immediately brighter, the increase of number
compensating for the decrease of brilliance.
Were the Universe infinitely extended, the whole
heavens would shine with the brilliance of the
Sun. Newcomb, therefore, concludes that " that
collection of stars which we call the Universe
is limited in extent."

Positive evidence that this is the case was
obtained by Giovanni Celoria, now director of

224 A CENTURY'S PROGRESS IN ASTRONOMY.

the Milan Observatory, in the course of a series
of star-gauges at the north galactic pole. Using
a small refractor, showing stars barely to the
eleventh magnitude, he found he could see
exactly the same number of stars as Herschel's
large reflector, indicating that increase of optical
power will not increase the number of stars
visible in that direction. Celoria's observation
can only be explained on the assumption that
the Universe is limited in extent, as otherwise
Herschel's telescope should have shown more
stars than Celoria's, even granting an extinction
of light, — a theory which Newcomb, Schiaparelli,
and others have shown to be quite untenable.
That the Universe is limited in extent is about
all that is known for certain, although even this
has been called in question, notably by E. W.
Maunder and H. H. Turner. The problem of
the construction of the heavens is by no means
solved, although several more or less probable
theories have been advanced.

A series of investigations on stellar distri-
bution, from 1884 to 1898, led Hugo Seeliger,
director of the Munich Observatory, to some
remarkable deductions. He believes the Uni-
verse to be flattened at the galactic poles. The
Galaxy is the zone of stellar condensation, and
he concludes the distance of the Solar System

STELLAR DISTRIBUTION. 225

from the inner border of the zone to be 500
times the distance of Sirius, while the external
border is 1100 times that distance. The Uni-
verse is finite in extent, its limits being about
9000 light years from the Solar System. In
Seeliger's opinion the extinction of light may
come into play beyond our Universe, and pre-
vent us seeing other collections of stars.

The question of external universes is purely a
hypothetical one, although there is undoubtedly
much to be said in its favour. These universes
have never been seen, and we can only speculate
as to their existence. The last word on the
subject is by Gore, in 1893, in his elaborate
work, * The Visible Universe/ He regards the
Solar System as a system of the first order,
and the Galaxy and its fellow-universes of the
second. He makes a calculation of the possible
distance of an external universe of his second
order. He assumes the distance of the nearest
universe from our Galaxy as proportional to
that separating the Sun from a Centauri,
and reaches the amazing conclusion that the
distance of the nearest Galaxy is no less than
520,149,600,000,000,000,000 miles, — a distance
which light, with its inconceivable velocity of
186,000 miles a second, would take almost
ninety millions of years to traverse.

p

226 A CENTURY'S PROGRESS IN ASTRONOMY.

These calculations absolutely overwhelm the
mind, which is unable to comprehend such vast
distances. Our universe is indeed, as Flammarion
expresses it, a point in the infinite. The cal-
culations of J. E. Gore represent our highest
scientific conception of the universe. He sums
up his investigations with the following words :
" Although we must consider the number of
visible stars as strictly finite, the numbers of
stars and systems really existing, but invisible
to us, may be practically infinite. Could we
speed our flight through space on angel wings
beyond the confines of our limited universe to
a distance so great that the interval which
separates us from the remotest fixed star might
be considered as merely a step on our celestial
journey, what further creations might not then
be revealed to our wondering vision ? Systems
of a higher order might there be unfolded to
our view, compared with which the whole of
our visible heavens might appear like a grain
of sand on the ocean shore, — systems perhaps
stretching out to infinity before us, and reach-
ing at last the glorious * mansions ' of the
Almighty, the Throne of the Eternal."

CHAPTER XIII.

CELESTIAL EVOLUTION.

IN the second chapter we outlined the nebular
hypothesis as propounded by Herschel. Some
time earlier the French mathematician, Laplace,
had put forward his theory of the evolution of
the Solar System. Pierre Simon Laplace was
born at Beaumont- en -Auge, near Honfleur, in
1749, and was educated in the Military School
of his native town. In 1767 he became Assist-
ant Professor of Mathematics at Beaumont,
and some years later at the Military School
in Paris, which position he retained for many
years. Member of the Institute and Minister
of the Interior under Napoleon, he was created
a Marquis by Louis XVIII., and died at Arcuile
on March 5, 1827.

In the last chapter of his popular work, the
' Systeme du Monde/ Laplace put forward his
nebular theory "with that distrust which every-
thing ought to inspire that is not the result of

228 A CENTURY'S PROGRESS IN ASTRONOMY.

observation or calculation." Laplace noticed
that in the Solar System all the planets re-
volved round the Sun in the same direction,
from west to east, and that the satellites of
the planets obeyed the same law. He also
observed that the Sun, Moon, and planets
rotated on their axes in the same direction as
they revolved round the Sun ; also that the
planets moved round the Sun, and the satellites
round their primaries, in almost the same plane
as the Earth's orbit, the plane of the ecliptic.
It was evident that these remarkable congruities
were not the result of chance, and accordingly
Laplace expressed his belief that the Solar
System originated from a great nebula, which
in condensing detached various rings in the
process of rotation. These rings condensed into
the various planets and their satellites.

Laplace's theory was powerfully supported by
Herschel's observations of the various nebulae
in the heavens. But, with the supposed resolu-
tion of the various nebulae after the erection of
the Rosse reflector in 1845, the evidence in
favour of the nebular theory seemed to be
greatly reduced. In 1864, however, the dis-
covery of the gaseous nebulae, by means of the
spectroscope, gave further support to the theory.
Powerful aid was lent to the nebular hypoth-

CELESTIAL EVOLUTION. 229

esis by the famous German physicist, Hermann
Ludwig Ferdinand von Helmholtz (1821-1894),
in 1854, in his theory of the maintenance of the
Sun's heat. Many theories had been already
advanced to account for this. After the dis-
covery of the conservation of energy, Julius
Robert Mayer, one of the discoverers, put
forward the theory that the Solar heat was
sustained by the inflow of meteorites from space,
and this idea was developed in 1854 by Sir
William Thomson, now Lord Kelvin (born
1824), but it was soon apparent that the
supply of meteors required to sustain the Solar
heat was such as would have increased the
mass of the Sun very considerably. Accord-
ingly the hypothesis was partially abandoned,
and was succeeded by that of Helmholtz, who
pointed out that the radiation of the Sun's
heat was the result of its contraction through
cooling. The rate was then estimated at 380
feet yearly, or a second of arc in 6000 years.
This theory was at once generally accepted. It
assumes the Sun to be still contracting, and
therefore, on going backwards in imagination,
we reach a period when the Sun must have
been much larger than now, and, in fact,
extended beyond the orbit of Neptune.

Several objections to Laplace's nebular theory

230 A CENTURY'S PROGRESS IN ASTRONOMY.

were urged by various investigators. Among
these was the retrograde motions of the satellites
of Uranus and Neptune, and the extremly rapid
revolution of the inner satellite of Mars. Other
objections were urged by Babinet, Kirkwood, and
others, and at length a sweeping reform of the
nebular theory was proposed by Faye in 1884,
in his work, ' Sur 1'Origine du Monde/ Faye put
forward the idea that all the planets interior
to the orbit of Uranus were formed inside the
solar nebula, while Uranus and Neptune came
into existence after the development of the Sun
was far advanced. But the objections to Faye's
theory are formidable, and the hypothesis has
not been accepted.

A popular exposition of the nebular theory
was given in 1901 in Ball's work on 'The
Earth's Beginning/ He exhaustively discusses
the whole question, and explains the retrograde
motion of the satellites of Uranus and Neptune
as due to the fact that the planes of the orbits
of the satellites will eventually be brought to
coincide with the ecliptic. These motions, says
Ball, do not disprove the nebular theory. " They
rather illustrate the fact that the great evolu-
tion which has wrought the Solar System into
its present form has not finished its work : it
is still in progress."

CELESTIAL EVOLUTION. 231

The theory that the Sun's heat was main-
tained by meteors, was extended by Proctor in
1870 to explain the growth of the planets
through meteoric aggregation as well as nebular
condensation. Certainly the theory, as developed
by Proctor, accounted fairly well for the various
features of the Solar System ; but the highest
development of the meteoritic theory is due to
Lockyer, who published his views in 1890, in
his work, * The Meteoritic Hypothesis.' Lockyer
claims that his views are merely extensions of
Schiaparelli's ideas regarding the concentration
of celestial matter. He considered the chief
nebular line to be identical with the remnant
of the magnesium fluting, which is conspicuous
in cometic and meteoric spectra ; but Huggins
and Keeler, with more powerful instruments,
disproved the supposed coincidence. Lockyer
considers that " all self-luminous bodies in the
celestial space are composed either of swarms
of meteorites or of masses of meteoric vapour
produced by heat. The heat is brought about
by the condensation of meteor swarms, due to
gravity, the vapour being finally condensed into
a solid globe."

Lockyer divided the stars into seven groups,
according to temperature, the order of evolution
being from red stars through a division of second-

232 A CENTURY'S PROGRESS IN ASTRONOMY.

type stars to Sirian stars, regarded as the hottest
stars ; through a second division of solar stars
to fourth-type stars. In fact, the theory aspires
to give a complete explanation of all celestial
phenomena, from meteors to nebulae. Newcomb,
however, considers that the objections to the
theory are insuperable, and his opinion is shared
by the majority of astronomers, many of whom,
however, consider that there are elements of
truth in the theory ; but Lockyer undoubtedly
carried his ideas to an extravagant extent.

Lockyer's evolutionary order of the stars is
not supported by Vogel. Zollner suggested in
1865 that yellow and red stars are simply white
stars in a further stage of cooling ; but Angstrom
showed that atmospheric composition is a safer
criterion of age than colour. Vogel's classifica-
tion, first published in 1874, and further developed
in 1895, is from the standpoint of evolution. He
considers Orion stars and Sirian stars to be the
youngest orbs. Solar stars are considered by
Vogel to have wasted much of their store of
radiation, and red stars are viewed as " effete
suns, hastening rapidly down the road to final
extinction." He considers stars of Secchi's fourth
type to be also dying suns, both types represent-
ing alternative roads for stars of the Solar type
in their decline into dark stars. This view is

CELESTIAL EVOLUTION. 233

supported by Duner, and is distinctly confirmed
by Hale's observations with the Yerkes telescope.
Vogel's views, in fact, are generally accepted
among astronomers. The nebular theory, modi-
fied by subsequent research, seems destined to
hold its own against all attacks.

Distinctly supplementary to the nebular theory
are the remarkable researches, commenced in
1879, by Sir George Howard Darwin (born
1845), son of Charles Darwin the great biologist.
George Howard Darwin was born in 1845, at
Downe in Kent, was educated at Cambridge, and
studied for the law; but in 1873 he returned to
Cambridge, where he became Plumian Professor
of Astronomy in 1883. In 1879 he communicated
to the Royal Society the first of his papers on
tidal friction, which were summed up in his book
on 'The Tides/ published in 1898. He finds
that the tides act upon the Earth as a brake
does upon a machine, — they tend to retard its
rotation. Consequently, the day is growing
longer, the Moon's orbit is becoming enlarged,
and its period of revolution is being lengthened.

At present the day is about twenty-four hours
long, and the month about twenty-seven days.
The day, however, will be lengthened at a more
rapid rate than the month, and in the remote
future the day and month will both last fifty-five

234 A CENTURY'S PROGRESS IN ASTRONOMY.

of our present days. The Moon will revolve
round the Earth in the same period that the
Earth rotates on its axis, and the two bodies
will perform their circuit round the Sun as if
united by a bar.

Not only can we foresee the future of the
Earth-Moon System, but we can also read the
past. According to Darwin's theory, the Earth,
in the remote past, was probably rotating on its
axis in a very short period, between three and
five hours. The Moon must then have been
much nearer us than it is now, and was prob-
ably revolving round its primary in the same
period that the Earth took to rotate on its axis.
The two globes, then gaseous, must have been
revolving almost in actual contact. Had the
month been even a second shorter than the day,
the Moon must inevitably have fallen back on
the Earth. As it was, the condition of affairs
could not endure. The condition of the Moon
resembled that of an egg balanced on its point.
The Moon must either recede from the Earth or
fall back upon it. The solar tide here interfered,
and caused the Moon to recede from its primary
until it reached its present distance of 239,000
miles.

The fact that the Earth and Moon were almost
in contact suggests that they were probably in

CELESTIAL EVOLUTION. 235

contact. In other words, the Moon originally
formed part of the Earth, which, in consequence
of its short - rotation period, and probably also
owing to the interference of the solar tide, split
into two portions, and the smaller of these now
forms the Moon. It is likely that the matter
now forming the Moon was detached from the
Earth in separate particles. Just as the tides
raised by the Moon tend to retard the motion
of the Earth, so the Earth tides raised in the
Moon have already done their work. The Moon
now rotates on its axis in the same time as it
revolves round the Earth. Part of the evolu-
tion of the Earth -Moon system is completed.
Schiaparelli's discovery that the rotation periods
of both Venus and Mercury coincide with their
times of revolution is distinctly confirmatory of
Darwin's theory.

In his chapter on the " Evolution of Celestial
Systems " in his book on ' The Tides/ Darwin dis-
cusses the distribution of the satellites of the Solar
System. He says of the evolution of a planet :
"We have seen that rings should be shed from
the central nucleus when the contraction of the
nebula has induced a certain degree of augmen-
tation of rotation. Now, if the rotation were
retarded by some external cause, the genesis of
a ring might be retarded or entirely prevented."

236 A CENTURY'S PROGRESS IN ASTRONOMY.

He then remarks that probably the formation
of the Moon was retarded, and in the case of
Mercury and Venus, solar tidal friction pre-
vented satellite formation. This explains why
Mercury and Venus have no satellites, the
Earth only one, Mars two, while the exterior
planets have each several satellites.

The theory of tidal friction was extended in
1892 to the explanation of the double stars
by the American astronomer, See. See showed
by mathematical calculation the effects of tidal
friction in shaping the eccentric orbits of the
binary stars, the course of evolution being traced
from double stars, revolving almost in contact,
which the spectroscope reveals, to the tele-
scopic doubles. See's researches have done much
to supplement those of Darwin, who considers
that there are two types of cosmical evolution,
— the Laplacian, and the " second " or lunar
type.

Lowell, in his work on * The Solar System '
(1903), adds six congruities to those remarked
by Laplace and his successors. These are, "All
the satellites turn the same face to their
primaries (so far as we can judge) ; Mercury,
and probably Venus, do the same to the Sun ;
one law governs position and size in the Solar
System and in all the satellite systems ; orbital

CELESTIAL EVOLUTION. 23*7

inclinations in the satellite systems increase with
distance from the primary ; the outer planets
show a greater tilt of axis to orbit-plane with
increased distance from the Sun (so far as detect-
able) ; the inner planets show a similar relation."

The fate of the average solar star is sketched
out by Vogel's classification, and by any evolu-
tionary hypothesis which we may adopt. In the
words of Lowell : " Though we cannot as yet
review with the mind's eye our past, we can, to
an extent, foresee our future. We can with
scientific confidence look forward to a time when
each of the bodies composing our Solar System
shall turn an unchanging face in perpetuity to
the Sun. Each will then have reached the end
of its evolution set in the unchanging stare of
death. Then the Sun itself will go out, becom-
ing a cold and lifeless mass ; and the Solar
System will circle unseen, ghostlike, in space,
awaiting only the resurrection of another cosmic
catastrophe."

As to what this cosmic catastrophe will be,
science gives no definite idea ; nor can astrono-
mers say with certainty whether the Universe
will come to an end by the extinction of its
luminaries, or whether the suns and planets
will be brought back to luminosity again ; but
the human mind shrinks from the idea of a

238 A CENTURY'S PROGRESS IN ASTRONOMY.

dead Universe. At this point science has said
its last word, and must give place to religion.
In our day we may repeat with deeper mean-
ing the words of the Scottish astronomer,
Thomas Dick : " Here imagination must drop
its wing, since it can penetrate no further
into the dominions of Him who sits on the
Throne of Immensity. Overwhelmed with a
view of the magnificence of the Universe, and
of the perfections of its Almighty Author, we
can only fall prostrate in deep humility and
exclaim, ' Great and marvellous are Thy works,
Lord God Almighty.' "

INDEX.

Absolute parallax, 158.

Adams, J. C., 78, 116, 117, 118,

119, 120, 140.
Adams, W., 205.
Aerolites, 147, 148, 149.
Airy, Sir G. B., 27, 104, 117, 120.
Aitken, R. G., 202.
Alcyone (77 Tauri), 217.
Aldebaran (a Tauri), 151, 166, 170,

172.
Algol (/3 Persei), 178, 182, 183,

184, 193, 204.
Al-Sufi, 180, 183.
Altair (a Aquilse), 170.
Anderson, T. D., 191, 192.
Andromeda nebula, 180, 208.
Andromedse (7), 201.
Andromedse (Nova), 180.
Andromedid meteors, 142, 149.
Angstrom, A. J., 50, 51.
Antares (a Scorpii), 171.
Aquila, 195.
Aquilee (??), 185, 186.
Arago, F. J. D., 6, 11, 31, 37,

40, 118, 120, 129.
Arcturus (a Bootis), 165, 170.
Arequipa Observatory, 75.
Argelander, F. W. A., 27, 159,

167, 178, 179, 180, 218.
Argo Navis, 221.
Argus (TJ), 187, 188.
Armagh Observatory, 206.
Asteroids, 19, 62, 97-102.

Astronomer - Royal of Scotland,
134, 155, 191 ; of England, 59,
117 ; of Ireland, 151, 156.

Astronomy of the invisible, 203.

Aurigse (Nova), 191, 192, 195.

Auwers, A., 167, 188, 203.

Babinet, 230.

Baily, F., 159.

Bakhuyzen, H. G., 91.

Ball, Sir R. S., 23, 34, 108, 141,

149, 156, 158, 230.
Barnard, E. PI, 19, 95, 107, 108,

110, 111, 113, 136, 191, 211.
Beer, W., 68, 69, 90.
Bellatrix (7 Orionis), 209.
Belopolsky, A., 87, 110, 166, 185,

186, 204, 205.

Berlin Observatory, 119, 120.
Bessel, F. W., 24, 82, 116, 151,

152, 153, 154, 159, 167, 202,

203.
Betelgeux (a Orionis), 165, 171,

172, 182, 187.
Biela, W., 128.
Biela's comet, 128, 129, 142, 143,

146, 149.

Birmingham, J., 189.
Bode, J. E., 97, 98, 152.
Bode's Law, 97.
Boeddicker, 0., 77.
Bond, G. P., 103, 109, 130, 136,

207.

240

INDEX.

Bond, W. 0., 109, 112, 120.
'Bonn Durchmusterung,' 159,

160, 218.

Bonn Observatory, 88, 97, 160.
Bootis (c), 30.
Borisiak, 192.
Boss, L., 168.
Bouvard, A., 115, 116.
Bradley, J., 159, 167.
Bredikhine, T. A., 105, 131, 132,

133, 134, 135.

Brewster, Sir D., 50, 101, 178.
Brinkley, J., 151.
Briinnow, F., 156.
Bruno, G., 35.
Buffon, 103.
Bunsen, R. W., 51.
Burchell, 188.
Burnham, S. W., 201, 202, 212.

Callandreau, 0., 136.
Callandrelli, 151.
Cambridge Observatory, 116.
Campbell, T. (Poet), 2.
Campbell, W. W., 24, 107, 110,

166, 168, 187, 191, 193, 204,

205.

Canals of Mars, 91, 92, 93, 94, 95.
Cancri (C), 206.
Cancri (S), 180.
Canis Major, 188.
' Cape Durchmusterung,' 161, 162.
Cape Observatory, 155, 157.
Capella (a Auriga), 170, 176, 193,

205.

Camera, L., 100.
Carpenter, J., 73.
Carrington, R. C., 45, 46, 59.
Cassini, D., 21.
Cassiopeia, 221.
Castor (a Geminorum), 30, 200,

205.
Celoria, G., 202, 218, 221, 223,

224.

Centauri (a), 155, 188, 225.
Centaurus, 221.
Cephei (S), 178, 182, 185, 186.
Cepheus, 221.
Ceres, 19, 98, 101.

Cerulli, V., 86, 91, 94.

Chacornac, 161.

Challis, J., 116, 119, 120.

Chambers, G. F., 31.

Chandler, S. C., 88, 89, 181, 184.

Chladni, E., 138.

Chromosphere, solar, 55, 56.

Clark, A., 202.

Clerke, Miss A. M., 3, 5, 8, 12,
13, 15, 25, 26, 34, 42, 58, 75,
86, 92, 105, 109, 124, 125, 131,
132, 133, 140, 142, 169, 186,
187, 189.

Clerk-Maxwell, J., 109, 110.

Coggia's comet, 131, 132, 133.

Comet families, 135.

Comets, 24, 123-137, 141, 142,
143, 144, 146, 149, 152.

Common, A. A., 107, 209.

Copeland, R., 134, 135, 190, 208.

Cornu, A., 189.

Corona Borealis, 188.

Corona, solar, 55, 57, 64.

Coronse (Nova), 188, 189.

Crossley, E., 211.

Crux, 221.

Cygni (61), 152, 158.

Cygni (Y), 184, 185.

Cygni (Nova), 189, 190.

Cygnus, 152, 189, 221.

Damoiseau, 78.

D'Arrest, H. L., 96, 119, 142,

212.

Dartmouth Observatory, 56.
Darwin, Sir G. H., 233, 234, 235,

236.

Dawes, W. R., 90, 117.
De la Rue, W., 52, 75.
Delaunay, C. E., 78, 79.
Dembowski, E., 201.
Deneb (o Cygni), 165.
Denning, W. F., 84, 85, 91, 95,

105, 111, 112, 144, 145, 146.
Deslandres, H., 110.
Dick, T., 85, 238.
Disc-theory, 32, 36, 38, 39, 214,

218.
DiVico, F., 85, 86, 170.

INDEX.

241

Doberck, W., 201.

Donati, G. B., 130, 131, 169.

Donati's comet, 130, 133, 136.

Doppler, C., 57, 58.

Doppler's Principle, 58, 59, 87,

110, 165, 168, 203.
Douglass, A. E., 92, 107.
Draconis (\), 182.
Draper, H., 136, 172, 175.
Dreyer, J. L. E., 206.
Dunecht Observatory, 157.
Dune-r, N. C., 58, 59, 174, 175,

181, 184, 185, 201, 202, 233.
Dunkin, E., 27, 167.
Dunsink Observatory, 156.

Earth, 76, 97, 103, 104, 147, 148,

149, 153, 154, 156, 236.
Earth-Moon system, 234, 235.
Easton, C., 221.
Eclipses, lunar, 77.
Eclipses, solar, 56, 57, 80, 81.
Edinburgh (Royal) Observatory,

195.

Electrical repulsion theory, 126.
Elger, T. G., 74.
Elkin, W. L., 157.
Encke, J. F., 30, 61, 119, 127,

128, 216.

Encke's comet, 127, 128, 137.
Erman, 140.
Eros, 62, 101.
Ertborn, 85.
Euler, L., 88, 89.
Evolution, planetary, 228, 229,

230 231
Evolution, stellar, 33, 34, 231, 232.

Faye, H., 60, 129, 230.
Faye's comet, 129, 137.
Ferguson, J., 9, 178.
Flammarion, C., 87, 91, 95, 121,

147, 164, 187, 195, 201, 202,

226.

Flamsteed, J., 5.
Fleming, Mrs, 192, 195.
Forbes, G., 122.
Fraunhofer, J. 47, 48, 49, 50,

151, 153, 169.

Fraunhofer lines, 48, 49, 50, 51,

169, 172.
Frost, E. B., 205.

Galactic poles, 35, 224.
Galaxies, external, 32, 218, 225,

226.
Galaxy, or Milky Way, 32, 36-42,

186, 211, 215, 216, 217, 219,

220, 221; 224, 225.
Galileo, 44, 107.
Galle, J. G., 62, 108, 109, 119,

142.

Galloway, T., 167.
Gambart, 128.
Gauss, C. F., 27, 98, 167.
Gautier, A., 45.
Gemini, 11, 194.
Geminorum (Nova), 194.
Geminorum (0, 180, 182, 185.
George III., 11, 23.
Gill, Sir D., 62, 136, 155, 157,

160, 161, 221.
Glasgow Observatory, 216.
Goodricke, J., 178, 183.
Gore, J. E., 24, 38, 179, 181, 182,

183, 192, 202, 215, 220, 225,

226.
Gould, B. A., 135, 160, 163, 180,

220, 221.
Grant, E,., 216.
Gravitation, law of, 29.
Greenwich Observatory, 59, 117.
Grimmler, 192.
Groombridge (1830), 156, 162,

223.

Groombridge (1618), 156.
Gruithuisen, 87.

Hale, G. E., 55, 57, 233.

Hall, A., 96, 111, 112, 156, 190.

Hall, Maxwell, 121.

Halley, E., 138.

Halley's comet, 123, 130, 152.

Halm, J., 195, 196.

Hansen, P. A., 61, 78, 79.

Hansky, A., 57.

Harding, K. L., 99, 153, 220.

Hartwig, E., 190.

Q

242

INDEX.

Harvard Observatory, 174, 175,
191.

Hasselberg, B., 148, 190.

Heis, E., 179, 220.

Heliometer, 153, 157.

Helium stars, 174.

Helmholtz, H., 61, 229.

Hencke, K. L., 99.

Henderson, T., 154, 155.

Henry, Paul and Prosper, 100,
114, 210.

Hercules, 167.

Herculis (a), 182.

Herculis (A), 26.

Herschel, William, 1-42, 43, 60,
63, 65, 69, 74, 77, 85, 90, 99,
103, 109, 111, 112, 115, 123,
150, 162, 167, 176, 196, 197,
207, 214, 216, 218, 224, 227.

Herschel, A., 144.

Herschel, Caroline, 6, 8, 9, 12, 13,
14, 30, 35, 127, 198.

Herschel, Sir J., 4, 17, 27, 30, 37,
50, 112, 113, 120, 130, 144,
167, 187, 188, 197, 198, 199,
200, 214, 215.

Hind, J. R., 99, 129, 135, 180,
188.

Hoek, 135.

Holden, E. S., 191.

Hough, G., 105.

Houzeau, J. C., 220.

Huggins, Lady, 172.

Huggins, Sir W., 54, 57, 74, 95,
106, 114, 131, 136, 165, 170,
171, 172, 173, 189, 190, 191,
193, 195, 207, 208, 212, 231.

Humboldt, A., 44, 139.

Hussey, W. J., 116, 201.

Innes, R., 188.

Intra-Mercurial planet, 80, 81.
Italian spectroscopists, 54, 55.

Janssen, P. J. C., 52, 53, 54, 57,

59, 112, 209.
Juno, 19, 99, 101.
Jupiter, 20, 75, 97, 101-108, 112,

114, 121, 122, 135, 144, 146.

Juvisy Observatory, 164.

Kaestner, 65, 124.

Kaiser, F., 90.

Kant, I., 34, 35, 101, 103.

Kapteyn, J. C., 27, 158, 161, 162,

168, 221, 222.
Keeler, J. E., 95, 114, 185, 208,

211, 212, 213, 231.
Kelvin, Lord, 229.
Kempf, P., 181.
Kepler, J., 5, 35, 137.
Kirchoff, G. R., 61, 169, 172.
Kirkwood, D., 140, 230.
Klein, H. J., 73.
Klinkerfues, E., 142, 202.
Konkoly, N., 175.
Kiistner, F., 88.

Lalande, 23, 152.

Lambert, J. H., 34.

Lament, J., 44.

Langley, S. P., 77.

Laplace, P. S., 20, 33, 34, 77, 109,

148, 152, 195, 227, 228, 229.
Lassell, W., 112, 115, 117, 120.
Latitude, variation of, 88, 89.
Leipzig Observatory, 173.
Leonid meteors, 139, 140, 142.
Leonis (/3), 183.
Lescarbault, 80.
Le Verrier, U. J. J., 61, 80, 81,

118, 119, 120, 142, 164.
Ley den Observatory, 91.
Librae (S), 180.
Lick Observatory, 93, 107, 166,

168, 191, 213.
Light, extinction of, 40, 215, 216,

224, 225.

Lindsay, Lord, 157.
Linn^ 71, 72.
Lockyer, Sir J. N., 52, 53, 54, 55,

58, 149, 174, 191, 193, 195,

208, 209, 231, 232.
Lockyer, W. J. S., 210.
Loewy, M., 75.
Lohrmann, W. G., 68, 71.
Lohse, W. 0., 88, 105.
Loomis, 188.

INDEX.

243

Lowell, P., 83, 84, 86, 87, 91, 92,

93, 94, 122, 236, 237.
Lowell Observatory, 92, 94, 106,

114.

Lund Observatory, 59.
Lupus, 221.
Luther, R., 100.
Lyra, 221.

Lyra (/3), 178, 182, 185.
Lyrid meteors, 122.

Maclaurin, C., 9.

Maclear, Sir T., 155.

Madler, J. H., 27, 68, 69, 71, 96,

104, 202, 203, 216, 217, 218.
Magellanic clouds, 219.
Magnetism, 44, 60.
Mars, 18, 19, 90-97, 101, 144, 236.
Maunder, E. W., 59, 60, 94, 95,

134, 145, 166, 224.
Mascari, A., 86.
Mayer, C., 164.
Mayer, J. R., 229.
Mazapil meteorite, 149.
M^chain, 127.
Mee, A., 68.
Melloni, 76.

Mercury, 18, 80, 81-84, 97, 236.
Messier, C., 30.
Meteorites, 147, 148, 149, 229,

231.

'Meteoritic Hypothesis,' 231, 232.
Meteors, 138-149.
Meudon Observatory, 59.
Milan Observatory, 82, 202, 224.
Milky Way. See Galaxy.
Miller, W. A., 50, 172.
Mira Ceti, 11, 182, 186, 187.
Mitchel, 0. M., 31.
Mizar (CUrsie Majoris), 204.
Moller, A., 129.
Moon, the, 10, 24, 65-79, 90, 95,

148, 228.

Moscow Observatory, 132.
Mouchez, A., 161.
Miiller, G., 175, 181.
Munich Observatory, 44, 224.

Napoleon, 67, 127.

Nasmyth, J., 73, 103.
Nebulae, 30, 31, 207-213, 228.
Nebular Hypothesis, 33, 195, 227,

228, 229, 230, 233.
Neison (Nevill), E., 73.
Neisten, 86, 105.

Neptune, 120, 121, 135, 229, 230.
Newall, H. F., 205.
Newcomb, S., 27, 64, 78, 89, 94,

162, 168, 220, 222, 223, 224,

232.

Newton, H. A., 140, 141.
Newton, Sir I., 2, 17, 29, 77.
Nichol, J. P., 31, 207.
Nordvig, L., 192.

Olbers, H. W. M., 19, 20, 69, 98,
99, 123, 124, 125, 126, 127,
129, 130, 139, 148, 152, 153.

Olbers' comet, 125.

Olmsted, D., 138, 207.

Ophiuchi (a), 183.

Orion, 221.

Orion nebula, 10, 33, 207, 208,
209, 213.

Orion stars, 174, 193, 209, 232.

Orionis (/c), 209.

Orionis (??), 209.

Orionis (0), 209.

Orionis (U), 181, 182, 186, 187.

Palisa, J., 100.

Pallas, 19, 99, 101.

Parallax, solar, 61, 62, 63, 101.

Parallax, stellar, 150-158, 190.

Paris Congresses, 161.

Paris Observatory, 78, 118, 171.

Perrine, C. D., 81, 108, 194.

Perrine's comet, 136.

Perrotin, H., 86, 91, 100, 114.

Peck, W., 162.

Persei (Nova), 192, 193, 194, 195.

Perseid meteors, 122, 141.

Perseus, 192, 221.

Peters, C. H. F., 100.

Peters, C. A. F., 142, 153, 155,

202.
Photography, astronomical, 54,

56,57, 59,75, 81,94, 108, 113,

244

INDEX.

136, 158, 160, 161, 172, 175,

192, 193, 194, 203, 208, 209,

210, 211, 212.
Photometry, 176, 177.
Piazzi, G., 19, 20, 98, 150.
Pickering, E. C., 174, 175, 176,

177, 181, 182, 183, 185, 193,

194, 203, 204, 220, 221.
Pickering, W. H., 75, 76, 81, 91,

92, 93, 107, 113, 209.
Plana, G., 78, 79.
Pleiades, 124, 210, 217.
Pogson, N. R., 142, 180.
Pole Star, 205.
Pollux, 165, 170.
Pons, J. L., 127.
Pont^coulant, 79.
Potsdam Observatory, 46, 173,

176.

Pritchard, C., 158, 177.
Proctor, R. A., 4, 20, 38, 41, 90,

91, 104, 148, 163, 164, 218,

219, 220, 231.
Procyon (a Canis Minor is), 151,

203.
Prominences, solar, 52, 53, 55,

64.

Puiseux, P., 75.
Pulkowa Observatory, 200.

Quetelet, A., 139.

Radiant points, meteoric, 139,

144, 145, 146.
Ranyard, A. C., 106, 146.
Red spot on Jupiter, 105, 106.
Regulus (o Leonis), 164, 165.
Relative parallax, 157.
Roseau, Photospherique, 59.
Resisting medium, 128.
Respighi, L., 55.
Reversing layer, 56, 57.
Ricco, A., 87, 105.
Rigel (ft Orionis), 165, 209.
Ritchey, G., 194.
Roberts, A. W., 181.
Roberts, L, 209, 210.
Roche, E., 109.
Roman College Observatory, 85.

Rosse, third Earl of, 141, 156,

207, 208, 218.
Rosse, fourth Earl of, 77.
Rotation of the Sun, 58, 59 ; of

the planets, 82, 83, 84, 85, 86,

87, 104, 111, 112.
Rowland, H. A., 52.
Rutherfurd, L. M., 75, 169.

Sabine, Sir E., 44.

Safford, T. H., 202.

Santini, G., 159.

Savary, R, 30, 199.

Satellites, 96, 107, 108, 112, 113,

115, 120, 121, 236.
Saturn, 20, 21, 22, 97, 103, 108-

113, 121, 135.

Schaeberle, J. M., 93, 107, 191,

203.

Scheiner, C., 44.
Scheiner, J., 166, 174, 176.
Schiaparelli, G. V., 82, 83, 84,

85, 86, 87, 91, 92, 114, 141,
143, 149, 201, 220, 224, 231,
235

Schjellerup, H., 180.

Schmidt, J. F. J., 69, 70, 71, 72,

73, 104, 179, 180, 189.
Schonfeld, E., 160, 179, 180, 188,

189.
Schroter, J. H., 16, 65, 66, 67,

68, 69, 70, 74, 81, 82, 84, 85,

86, 87, 97, 99, 153.
Schwabe, S. H., 18, 43, 44, 46,

55.

Schwassman, A., 100.
Secchi, A., 52, 54, 55, 60, 72, 90,

114, 141, 170, 171, 173.
Secchi's types of stellar spectra,

170, 171, 173, 174, 175, 189,

232.

See, T. J. J., 201, 202, 236.
Seeliger, H., 110, 195, 196, 202,

206, 224, 225.
Serviss, G. P., 158.
Sirius (a Canis Majoris), 151, 170,

173, 188, 202, 225.
Slipher, V. M., 106, 114, 204.
Sime, J., 27.

INDEX.

245

Sola, J. C., 112.

Solar cluster, 221, 222.

Solar system, motion of, 26, 27,

167, 168.

South, Sir J., 198.
Spectroscopic binaries, 203, 204,

205.

Spencer, H., 218.
Spica (a Virginis), 204.
Sparer, F. W. G., 45, 46, 54, 59.
Star-catalogues, 159, 160, 161,

162,

Star-clusters, 30, 31, 32, 206, 210.
Star-drift, 164.
Star-gauging, 36, 40, 41, 224.
Stars, distance of, 150-158.
Stars, distribution of, 35, 39, 40,

198-214.
Stars, double, 28, 29, 30, 197-

206.

Stars, gaseous, 171, 174.
Stars, proper motion of, 162, 163,

164, 165.

Stars, radial motion of, 165, 166.
Stars, temporary, 156, 182, 188-

196.

Stars, triple and multiple, 206.
Stars, variable, 177-188.
Stellar spectra, 169-176, 187, 189,

190, 191, 193, 194.
Stellar universe, 35-42, 214, 215-

226.

Stereo-comparator, 100, 101.
Stokes, Sir G., 50.
Stone, E. J., 157, 160.
Stroobant, P., 88.
Struve, F. G. W., 3, 37, 38, 40,

42, 128, 151, 153, 200, 214,

215, 216, 218.
Struve, H., 201.
Struve, L., 27, 163, 167.
Struve, 0. W., 27, 110, 115, 120,

153, 156, 163, 200, 201.
Stumpe, O., 167.
Sun, 15, 16, 17, 40, 43-64, 65, 80,

81, 105, 125, 128, 170, 222,

228, 229, 230, 237.
Swift, L., 81.
Swift's comet, 136.

Tacchini, P., 55, 86, 87.

Taurus, 217, 221.

Tempel, E., 210.

Tennyson, 96.

Tidal friction, 79, 87, 233, 234,

235, 236.

Tisserand, F. F., 146.
Todd, D. P., 122.
Trans-Neptunian planet, 121, 122.
Trouvelot, E., 86, 87.
Tschermak, 148.
Tulse Hill Observatory, 171.
Turner, H. H., 194, 224.
Twining, A. C., 139.

Upsala Observatory, 59.
Uranometria Argentina, 160.
Uranus, 11, 20, 22, 23, 97, 113,

114, 115, 118, 121, 135, 141,

230.

Ursa Major, 162, 164.
Ursse Majoris (5), 182.
Ursse Majoris (|), 199.

Venus, 18, 84-88, 97, 235, 236.

Venus, transits of, 61, 62, 87.

Vega (a Lyr^e), 151, 165, 170, 172,
173.

Very, F. W., 77.

Vesta, 19, 99, 101, 102.

Vogel, H. C., 84, 88, 95, 102,
106, 114, 131, 148, 166, 173,
174, 175, 183, 184, 185, 190,
191, 193, 195, 204, 209, 232,
233, 237.

Vulcan, 81.

Washington Observatory, 96, 223.
Watson, J. C., 81, 100.
Webb, T. W., 72, 73, 104.
Weinek, L., 75.
Weiss, E., 142.
Well's comet, 134.
Whewell, W., 318.
Williams, A. S., 110, 193.
Wilson, A., 16.
Winlock, J., 177.
Winnecke, F. A. T., 131.
Witt, K. G., 101.

246 INDEX.

Wolf, Max, 100, 181, 191, 194, Yerkes Observatory, 55, 111, 202.

211. Young, C. A., 54, 56, 57, 58, 60,

Wolf, R., 44, 45, 188. 87, 114, 190.
Wolf and Rayet, 171.

Wolf-Rayet stars, 171, 174. Zach, F. X., 97, 98, 152.

Wollaston, W. H., 48. Zantedeschi, 77.

Wright, T., 34, 110. Zenger, 85, 88.

Zollner, J. C. F., 54, 58, 60, 84,

Yale Observatory, 157. 103, 132, 232.

CORRIGENDA.

P. 30, 1. 5, for " objects " read " orbits."

P. 36, 1. 13, for " unable " read " able."

P. 61, 1. 17, for " 8".371 " read » 8".571."

P. 63, 1. 21, for " bases " read " gases."

P. 100, 1. 16, for " Schwussmann " read " Schwassmann."

P. 167, 1. 28, for "Strumpe" read "Stumpe."

P. 184, 1. 11, for " star- variables " read "variable stars."

P. 199, 1. 23, for "2102" read "1202."

THE END.

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