_SOLAR OBSERVATIONS AND THEORIES_
The zeal with which solar studies have been pursued during the last half
century has already gone far to redeem the neglect of the two preceding
ones. Since Schwabe's discovery was published in 1851, observers have
multiplied, new facts have been rapidly accumulated, and the previous
comparative quiescence of thought on the great subject of the
constitution of the sun, has been replaced by a bewildering variety of
speculations, conjectures, and more or less justifiable inferences. It
is satisfactory to find this novel impulse not only shared, but to a
large extent guided, by our countrymen.
William Rutter Dawes, one of many clergymen eminent in astronomy,
observed, in 1852, with the help of a solar eye-piece of his own
devising, some curious details of spot-structure.[405] The
umbra--heretofore taken for the darkest part of the spot--was seen to be
suffused with a mottled, nebulous illumination, in marked contrast with
the striated appearance of the penumbra; while through this "cloudy
stratum" a "black opening" permitted the eye to divine farther
unfathomable depths beyond. The _hole_ thus disclosed--evidently the
true nucleus--was found to be present in all considerable, as well as in
many small maculæ.
Again, the whirling motions of some of these objects were noticed by
him. The remarkable form of one sketched at Wateringbury, in Kent,
January 17, 1852, gave him the means of detecting and measuring a
rotatory movement of the whole spot round the black nucleus at the rate
of 100 degrees in six days. "It appeared," he said, "as if some
prodigious ascending force of a whirlwind character, in bursting through
the cloudy stratum and the two higher and luminous strata, had given to
the whole a movement resembling its own."[406] An interpretation
founded, as is easily seen, on the Herschelian theory, then still in
full credit.
An instance of the same kind was observed by Mr. W. R. Birt in
1860,[407] and cyclonic movements are now a recognised feature of
sun-spots. They are, however, as Father Secchi[408] concluded from his
long experience, but temporary and casual. Scarcely three per cent. of
all spots visible exhibit the spiral structure which should invariably
result if a conflict of opposing, or the friction of unequal, currents
were essential, and not merely incidental to their origin. A whirlpool
phase not unfrequently accompanies their formation, and may be renewed
at periods of recrudescence or dissolution; but it is both partial and
inconstant, sometimes affecting only one side of a spot, sometimes
slackening gradually its movement in one direction, to resume it, after
a brief pause, in the opposite. Persistent and uniform notions, such as
the analogy of terrestrial storms would absolutely require, are not to
be found. So that the "cyclonic theory" of sun-spots, suggested by
Herschel in 1847,[409] and urged, from a different point of view, by
Faye in 1872, may be said to have completely broken down.
The drift of spots over the sun's surface was first systematically
investigated by Carrington, a self-constituted astronomer, gifted with
the courage and the instinct of thoughtful labour.
Born at Chelsea in May, 1826, Richard Christopher Carrington entered
Trinity College, Cambridge, in 1844. He was intended for the Church, but
Professor Challis's lectures diverted him to astronomy, and he resolved,
as soon as he had taken his degree, to prepare, with all possible
diligence, to follow his new vocation. His father, who was a brewer on a
large scale at Brentford, offered no opposition; ample means were at his
disposal; nevertheless, he chose to serve an apprenticeship of three
years as observer in the University of Durham, as though his sole object
had been to earn a livelihood. He quitted the post only when he found
that its restricted opportunities offered no farther prospect of
self-improvement.
He now built an observatory of his own at Redhill in Surrey, with the
design of completing Bessel's and Argelander's survey of the northern
heavens by adding to it the circumpolar stars omitted from their view.
This project, successfully carried out between 1854 and 1857, had
another and still larger one superposed upon it before it had even begun
to be executed. In 1852, while the Redhill Observatory was in course of
erection, the discovery of the coincidence between the sun-spot and
magnetic periods was announced. Carrington was profoundly interested,
and devoted his enforced leisure to the examination of records, both
written and depicted, of past solar observations. Struck with their
fragmentary and inconsistent character, he resolved to "appropriate," as
he said, by "close and methodical research," the eleven-year period next
ensuing.[410] He calculated rightly that he should have the field pretty
nearly to himself; for many reasons conspire to make public
observatories slow in taking up new subjects, and amateurs with freedom
to choose, and means to treat them effectually, were scarcer then than
they are now.
The execution of this laborious task was commenced November 9, 1853. It
was intended to be merely a _parergon_--a "second subject," upon which
daylight energies might be spent, while the hours of night were reserved
for cataloguing those stars that "are bereft of the baths of ocean." Its
results, however, proved of the highest interest, although the
vicissitudes of life barred the completion, in its full integrity, of
the original design. By the death, in 1858, of the elder Carrington, the
charge of the brewery devolved upon his son; and eventually absorbed so
much of his care that it was found advisable to bring the solar
observations to a premature close, on March 24, 1861.
His scientific life may be said to have closed with them. Attacked four
years later with severe, and, in its results, permanent illness, he
disposed of the Brentford business, and withdrew to Churt, near Farnham,
in Surrey. There, in a lonely spot, on the top of a detached conical
hill known as the "Devil's Jump," he built a second observatory, and
erected an instrument which he was no longer able to use with pristine
effectiveness; and there, November 27, 1875, he died of the rupture of a
blood vessel on the brain, before he had completed his fiftieth
year.[411]
His observations of sun-spots were of a geometrical character. They
concerned positions and movements, leaving out of sight physical
peculiarities. Indeed, the prudence with which he limited his task to
what came strictly within the range of his powers to accomplish, was one
of Carrington's most valuable qualities. The method of his observations,
moreover, was chosen with the same practical sagacity as their objects.
As early as 1847, Sir John Herschel had recommended the daily
self-registration of sun-spots,[412] and he enforced the suggestion,
with more immediate prospect of success, in 1854.[413] The art of
celestial photography, however, was even then in a purely tentative
stage, and Carrington wisely resolved to waste no time on dubious
experiments, but employ the means of registration and measurement
actually at his command. These were very simple, yet very effective. To
the "helioscope" employed by Father Scheiner[414] two centuries and a
quarter earlier, a species of micrometer was added. The image of the sun
was projected upon a screen by means of a firmly-clamped telescope, in
the focus of which were placed two cross-wires forming angles of 45°
with the meridian. The six instants were then carefully noted at which
these were met by the edges of the disc as it traversed the screen, and
by the nucleus of the spot to be measured.[415] A short process of
calculation then gave the exact position of the spot as referred to the
sun's centre.
From a series of 5,290 observations made in this way, together with a
great number of accurate drawings, Carrington derived conclusions of
great importance on each of the three points which he had proposed to
himself to investigate. These were: the law of the sun's rotation, the
existence and direction of systematic currents, and the distribution of
spots on the solar surface.
Grave discrepancies were early perceived to exist between determinations
of the sun's rotation by different observers. Galileo, with "comfortable
generality," estimated the period at "about a lunar month";[416]
Scheiner, at twenty-seven days.[417] Cassini, in 1678, made it 25·58;
Delambre, in 1775, no more than twenty-five days. Later inquiries
brought these divergences within no more tolerable limits. Laugier's
result of 25·34 days--obtained in 1841--enjoyed the highest credit, yet
it differed widely in one direction from that of Böhm (1852), giving
25·52 days, and in the other from that of Kysæus (1846), giving 25·09
days. Now the cause of these variations was really obvious from the
first, although for a long time strangely overlooked. Scheiner pointed
out in 1630 that different spots gave different periods, adding the
significant remark that one at a distance from the solar equator
revolved more slowly than those nearer to it.[418] But the hint was
wasted. For upwards of two centuries ideas on the subject were either
retrograde or stationary. What were called the "proper motions" of spots
were, however, recognised by Schröter,[419] and utterly baffled
Laugier,[420] who despaired of obtaining any concordant result as to the
sun's rotation except by taking the mean of a number of discordant ones.
At last, in 1855, a valuable course of observations made at Capo di
Monte, Naples, in 1845-6, enabled C. H. F. Peters[421] to set in the
clearest light the insecurity of determinations based on the assumption
of fixity in objects plainly affected by movements uncertain both in
amount and direction.
Such was the state of affairs when Carrington entered upon his task.
Everything was in confusion; the most that could be said was that the
confusion had come to be distinctly admitted and referred to its true
source. What he discovered was this: that the sun, or at least the outer
shell of the sun visible to us, has _no single period of rotation_, but
drifts round, carrying the spots with it, at a rate continually
accelerated from the poles to the equator. In other words, the time of
axial revolution is shortest at the equator and lengthens with increase
of latitude. Carrington devised a mathematical formula by which the rate
or "law" of this lengthening was conveniently expressed; but it was a
purely empirical one. It was a concise statement, but implied no
physical interpretation. It summarised, but did not explain the facts.
An assumed "mean period" for the solar rotation of 25·38 days
(twenty-five days nine hours, very nearly), was thus found to be
_actually_ conformed to only in two parallels of solar latitude (14°
north and south), while the equatorial period was slightly less than
twenty-five, and that of latitude 50° rose to twenty-seven days and a
half.[422] These curious results gave quite a new direction to ideas on
solar physics.
The other two "elements" of the sun's rotation were also ascertained by
Carrington with hitherto unattained precision. He fixed the inclination
of its axis to the ecliptic at 82° 45'; the longitude of the ascending
node at 73° 40' (for the epoch 1850 A.D.). These data--which have
scarcely yet been improved upon--suffice to determine the position in
space of the sun's equator. Its north pole is directed towards a star in
the coils of the Dragon, midway between Vega and the Pole-star; its
plane intersects that of the earth's orbit in such a way that our planet
finds itself in the same level on or about the 3rd of June and the 5th
of December, when any spots visible on the disc cross it in apparently
straight lines. At other times, the paths pursued by them seem
curved--downward (to an observer in the northern hemisphere) between
June and December, upward between December and June.
A singular peculiarity in the distribution of sun-spots emerged from
Carrington's studies at the time of the minimum of 1856. Two broad belts
of the solar surface, as we have seen, are frequented by them, of which
the limits may be put at 6° and 35° of north and south latitude.
Individual equatorial spots are not uncommon, but nearer to the poles
than 35° they are a rare exception. Carrington observed--as an extreme
instance--in July, 1858, one in south latitude 44°; and Peters, in June,
1846, watched, during several days, a spot in 50° 24' north latitude.
But beyond this no true macula has ever been seen; for Lahire's reported
observation of one in latitude 70° is now believed to have had its place
on the solar globe erroneously assigned; and the "veiled spots"
described by Trouvelot in 1875[423] as occurring within 10° of the pole
can only be regarded as, at the most, the same kind of disturbance in an
undeveloped form.
But the novelty of Carrington's observations consisted in the detection
of certain changes in distribution concurrent with the progress of the
eleven-year period. As the minimum approached, the spot-zones contracted
towards the equator, and there finally vanished; then, as if by a fresh
impulse, spots suddenly reappeared in high latitude, and spread
downwards with the development of the new phase of activity. Scarcely
had this remark been made public,[424] when Wolf[425] found a
confirmation of its general truth in Böhm's observations during the
years 1833-36; and a perfectly similar behaviour was noted both by
Spörer and Secchi at the minimum epoch of 1867. The ensuing period gave
corresponding indications; and it may now be looked upon as established
that the spot-zones close in towards the equator with the advance of
each cycle, their activity culminating, as a rule, in a mean latitude of
about 16°, and expiring when it is reduced to 6°. Before this happens,
however, a completely new disturbance will have manifested itself some
35° north and south of the equator, and will have begun to travel over
the same course as its predecessor. Each series of sun-spots is thus, to
some extent, overlapped by the succeeding one; so that while the average
interval from one maximum to the next is eleven years, the period of
each distinct wave of agitation is twelve or fourteen.[426] Curious
evidence of the retarded character of the maximum of 1883-4 was to be
found in the unusually low latitude of the spot-zones when it occurred.
Their movement downward having gone on regularly while the crisis was
postponed, its final symptoms were hence displaced locally as well as in
time. The "law of zones" was duly obeyed at the minima of 1890[427] and
1901, and Spörer found evidence of conformity to it so far back as
1619.[428] His researches, however, also showed that it was in abeyance
during some seventy years previously to 1716, during which period
sun-spots remained persistently scarce, and auroral displays were feeble
and infrequent even in high northern latitudes. An unaccountable
suspension of solar activity is, in fact, indicated.[429]
Gustav Spörer, born at Berlin in 1822, began to observe sun-spots with
the view of assigning the law of solar rotation in December, 1860. His
assiduity and success with limited means attracted attention, and a
Government endowment was procured for his little solar observatory at
Anclam, in Pomerania, the Crown Prince (afterwards Emperor Frederick)
adding a five-inch refractor to its modest equipment. Unaware of
Carrington's discovery (not made known until January, 1859), he arrived
at and published, in June, 1861,[430] a similar conclusion as to the
equatorial quickening of the sun's movement on its axis. Appointed
observer in the new Astrophysical establishment at Potsdam in 1874, he
continued his sun-spot determinations there for twenty years, and died
July 7, 1895.
The time had now evidently come for a fundamental revision of current
notions respecting the nature of the sun. Herschel's theory of a cool,
dark, habitable globe, surrounded by, and protected against, the
radiations of a luminous and heat-giving envelope, was shattered by the
first _dicta_ of spectrum analysis. Traces of it may be found for a few
years subsequent to 1859,[431] but they are obviously survivals from an
earlier order of ideas, doomed to speedy extinction. It needs only a
moment's consideration of the meaning at last found for the Fraunhofer
lines to see the incompatibility of the new facts with the old
conceptions. They implied not only the presence near the sun, as glowing
vapours, of bodies highly refractory to heat, but that these glowing
vapours formed the relatively cool envelope of a still hotter internal
mass. Kirchhoff, accordingly, included in his great memoir "On the Solar
Spectrum," read before the Berlin Academy of Sciences, July 11, 1861, an
exposition of the views on the subject to which his memorable
investigations had led him. They may be briefly summarised as follows:
Since the body of the sun gives a continuous spectrum, it must be either
solid or liquid,[432] while the interruptions in its light prove it to
be surrounded by a complex atmosphere of metallic vapours, somewhat
cooler than itself. Spots are simply clouds due to local depressions of
temperature, differing in no respect from terrestrial clouds except as
regards the kinds of matter composing them. These _sun-clouds_ take
their origin in the zones of encounter between polar and equatorial
currents in the solar atmosphere.
This explanation was liable to all the objections urged against the
"cumulus theory" on the one hand, and the "trade-wind theory" on the
other. Setting aside its propounder, it was consistently upheld perhaps
by no man eminent in science except Spörer; and his advocacy of it
proved ineffective to secure its general adoption.
M. Faye, of the Paris Academy of Sciences, was the first to propose a
coherent scheme of the solar constitution covering the whole range of
new discovery. The fundamental ideas on the subject now in vogue here
made their first connected appearance. Much, indeed, remained to be
modified and corrected; but the transition was finally made from the old
to the new order of thought. The essence of the change may be conveyed
in a single sentence. The sun was thenceforth regarded, not as a mere
heated body, or--still more remotely from the truth--as a cool body
unaccountably spun round with a cocoon of fire, but as a vast
_heat-radiating machine_. The terrestrial analogy was abandoned in one
more particular besides that of temperature. The solar system of
circulation, instead of being adapted, like that of the earth, to the
distribution of heat received from without, was seen to be directed
towards the transportation towards the surface of the heat contained
within. Polar and equatorial currents, tending to a purely superficial
equalisation of temperature, were replaced by vertical currents bringing
up successive portions of the intensely heated interior mass, to
contribute their share in turn to the radiation into space which might
be called the proper function of a sun.
Faye's views, which were communicated to the Academy of Sciences,
January 16, 1865,[433] were avowedly based on the anomalous mode of
solar rotation discovered by Carrington. This may be regarded either as
an acceleration increasing from the poles to the equator, or as a
retardation increasing from the equator to the poles, according to the
rate of revolution we choose to assume for the unseen nucleus. Faye
preferred to consider it a retardation produced by ascending currents
continually left behind as the sphere widened in which the matter
composing them was forced to travel. He further supposed that the depth
from which these vertical currents rose, and consequently the amount of
retardation effected by their ascent to the surface, became
progressively greater as the poles were approached, owing to the
considerable flattening of the spheroidal surface from which they
started;[434] but the adoption of this expedient has been shown to
involve inadmissible consequences.
The extreme internal mobility betrayed by Carrington's and Spörer's
observations led to the inference that the matter composing the sun was
mainly or wholly gaseous. This had already been suggested by Father
Secchi[435] a year earlier, and by Sir John Herschel in April,
1864;[436] but it first obtained general currency through Faye's more
elaborate presentation. A physical basis was afforded for the view by
Cagniard de la Tour's experiments in 1822,[437] proving that, under
conditions of great heat and pressure, the vaporous state was compatible
with a very considerable density. The position was strengthened when
Andrews showed, in 1869,[438] that above a fixed limit of temperature,
varying for different bodies, true liquefaction is impossible, even
though the pressure be so tremendous as to retain the gas within the
same space that enclosed the liquid. The opinion that the mass of the
sun is gaseous now commands a very general assent; although the gaseity
admitted is of such a nature as to afford the consistence rather of
honey or pitch than of the aeriform fluids with which we are familiar.
On another important point the course of subsequent thought was
powerfully influenced by Faye's conclusions in 1865. Arago somewhat
hastily inferred from experiments with the polariscope the wholly
gaseous nature of the visible disc of the sun. Kirchhoff, on the
contrary, believed (erroneously, as we now know) that the brilliant
continuous spectrum derived from it proved it to be a white-hot solid or
liquid. Herschel and Secchi[439] indicated a cloud-like structure as
that which would best harmonise the whole of the evidence at command.
The novelty introduced by Faye consisted in regarding the photosphere no
longer "as a defined surface, in the mathematical sense, but as a limit
to which, in the general fluid mass, ascending currents carry the
physical or chemical phenomena of incandescence."[440] Uprushing floods
of mixed vapours with strong affinities--say of calcium or sodium and
oxygen--at last attain a region cool enough to permit their combination;
a fine dust of solid or liquid compound particles (of lime or soda, for
example) there collects into the photospheric clouds, and descending by
its own weight in torrents of incandescent rain, is dissociated by the
fierce heat below, and replaced by ascending and combining currents of
similar constitution.
This first attempt to assign the part played in cosmical physics by
chemical affinities was marked by the importation into the theory of the
sun of the now familiar phrase _dissociation_. It is indeed tolerably
certain that no such combinations as those contemplated by Faye occur at
the photospheric level, since the temperature there must be enormously
higher than would be needed to reduce all metallic earths and oxides;
but molecular changes of some kind, dependent perhaps in part upon
electrical conditions, in part upon the effects of radiation into space,
most likely replace them. The conjecture was emitted by Dr. Johnstone
Stoney in 1867[441] that the photospheric clouds are composed of
carbon-particles precipitated from their mounting vapour just where the
temperature is lowered by expansion and radiation to the boiling-point
of that substance. But this view, though countenanced by Ångström,[442]
and advocated by Hastings of Baltimore,[443] and other authorities,[444]
is open to grave objections.[445]
In Faye's theory, sun-spots were regarded as simply breaks in the
photospheric clouds, where the rising currents had strength to tear them
asunder. It followed that they were regions of increased heat--regions,
in fact, where the temperature was too high to permit the occurrence of
the precipitations to which the photosphere is due. Their obscurity was
attributed, as in Dr. Brester's more recent _Théorie du Soleil_, to
deficiency of emissive power. Yet here the verdict of the spectroscope
is adverse and irreversible.
After every deduction, however, has been made, we still find that
several ideas of permanent value were embodied in this comprehensive
sketch of the solar constitution. The principal of these were; first,
that the sun is a mainly gaseous body; secondly, that its stores of heat
are rendered available at the surface by means of vertical
convection-currents--by the bodily transport, that is to say, of
intensely hot matter upward, and of comparatively cool matter downward;
thirdly, that the photosphere is a surface of condensation, forming the
limit set by the cold of space to this circulating process, and that a
similar formation must attend, at a certain stage, the cooling of every
cosmical body.
To Warren de la Rue belongs the honour of having obtained the earliest
results of substantial value in celestial photography. What had been
done previously was interesting in the way of promise, but much could
not be claimed for it as actual performance. Some "pioneering
experiments" were made by Dr. J. W. Draper of New York in 1840,
resulting in the production of a few "moon-pictures" one inch in
diameter;[446] but slight encouragement was derived from them, either to
himself or others. Bond of Cambridge (U.S.), however, secured in 1850
with the Harvard 15-inch refractor that daguerreotype of the moon with
which the career of extra-terrestrial photography may be said to have
formally opened. It was shown in London at the Great Exhibition of 1851,
and determined the direction of De la Rue's efforts. Yet it did little
more than prove the art to be a possible one.
Warren de la Rue was born in Guernsey in 1815, and died in London April
19, 1889. Educated at the École Sainte-Barbe in Paris, he made a large
fortune as a paper manufacturer in England, and thus amply and early
provided the material supplies for his scientific campaign. Towards the
end of 1853 he took some successful lunar photographs. They were
remarkable as the first examples of the application to astronomical
light-painting of the collodion process, invented by Archer in 1851; and
also of the use of reflectors (De la Rue's was one of thirteen inches,
constructed by himself) for that kind of work. The absence of a driving
apparatus was, however, very sensibly felt; the difficulty of moving the
instrument by hand so as accurately to follow the moon's apparent motion
being such as to cause the discontinuance of the experiments until 1857,
when the want was supplied. De la Rue's new observatory, built in that
year at Cranford, was expressly dedicated to celestial photography; and
there he applied to the heavenly bodies the stereoscopic method of
obtaining relief, and turned his attention to the delicate business of
photographing the sun.
A solar daguerreotype was taken at Paris, April 2, 1845,[447] by
Foucault and Fizeau, acting on a suggestion from Arago. But the attempt,
though far from being unsuccessful, does not, at that time, seem to have
been repeated. Its great difficulty consisted in the enormous
light-power of the object to be represented, rendering an inconceivably
short period of exposure indispensable, under pain of getting completely
"burnt-up" plates. In 1857 De la Rue was commissioned by the Royal
Society to construct an instrument specially adapted to the purpose for
the Kew Observatory. The resulting "photoheliograph" may be described as
a small telescope (of 3-1/2 inches aperture and 50 focus), with a
plate-holder at the eye-end, guarded in front by a spring-slide, the
rapid movement of which across the field of view secured for the
sensitive plate a virtually instantaneous exposure. By its means the
first solar light-pictures of real value were taken, and the autographic
record of the solar condition recommended by Sir John Herschel was
commenced and continued at Kew during fourteen years--1858-72. The work
of photographing the sun is now carried on in every quarter of the
globe, from Mauritius to Massachusetts, and the days are few indeed on
which the self-betrayal of the camera can be evaded by our chief
luminary. In the year 1883 the incorporation of Indian with Greenwich
pictures afforded a record of the state of the solar surface on 340
days; and 364 were similarly provided for in 1897 and 1899.
The conclusions arrived at by photographic means at Kew were
communicated to the Royal Society in a series of papers drawn up jointly
by De la Rue, Balfour Stewart, and Benjamin Loewy, in 1865 and
subsequent years. They influenced materially the progress of thought on
the subject they were concerned with.
By its rotation the sun itself offers opportunities for bringing the
stereoscope to bear upon it. Two pictures, taken at an interval of
twenty-six minutes, show just the amount of difference needed to give,
by their combination, the maximum effect of solidity.[448] De la Rue
thus obtained, in 1861, a stereoscopic view of a sun-spot and
surrounding faculæ, representing the various parts in their true mutual
relations. "I have ascertained in this way," he wrote,[449] "that the
faculæ occupy the highest portions of the sun's photosphere, the spots
appearing like holes in the penumbræ, which appeared lower than the
regions surrounding them; in one case, parts of the faculæ were
discovered to be sailing over a spot apparently at some considerable
height above it." Thus Wilson's inference as to the depressed nature of
spots received, after the lapse of not far from a century, proof of the
most simple, direct, and convincing kind. A careful application of
Wilson's own geometrical test gave results only a trifle less decisive.
Of 694 spots observed, 78 per cent. showed, as they traversed the disc,
the expected effects of perspective;[450] and their absence in the
remaining 22 per cent. might be explained by internal commotions
producing irregularities of structure. The absolute depth of
spot-cavities--at least of their sloping sides--was determined by Father
Secchi through measurement of the "parallax of profundity"[451]--that
is, of apparent displacements attendant on the sun's rotation, due to
depression below the sun's surface. He found that in every case it fell
short of 4,000 miles, and averaged not more than 1,321, corresponding,
on the terrestrial scale, to an excavation in the earth's crust of 1-1/5
miles. Of late, however, the reality of even this moderate amount of
depression has been denied. Mr. Howlett's persevering observations,
extending over a third of a century, the results of which were presented
to the Royal Astronomical Society in December, 1894,[452] availed to
shatter the consensus of opinion which had so long been maintained on
the subject of spot-structure.[453] It has become impossible any longer
to hold that it is uniformly cavernous; and what seem like actually
protruding umbræ are occasionally vouched for on unimpeachable
authority.[454] We can only infer that the forms of sun-spots are really
more various than had been supposed; that they are peculiarly subject to
disturbance; and that the level of the nuclei may rise and fall during
the phases of commotion, like lavas within volcanic craters.
The opinion of the Kew observers as to the nature of such disturbances
was strongly swayed by another curious result of the "statistical
method" of inquiry. They found that of 1,137 instances of spots
accompanied by faculæ, 584 had those faculæ chiefly or entirely on the
left, 508 showed a nearly equal distribution, while 45 only had faculous
appendages mainly on the right side.[455] Now the rotation of the sun,
as we see it, is performed from left to right; so that the marked
tendency of the faculæ was a lagging one. This was easily accounted for
by supposing the matter composing them to have been flung upwards from a
considerable depth, whence it would reach the surface with the lesser
_absolute_ velocity belonging to a smaller circle of revolution, and
would consequently fall behind the cavities or "spots" formed by its
abstraction. An attempt, it is true, made by M. Wilsing at Potsdam in
1888[456] to determine the solar rotation from photographs of faculæ had
an outcome inconsistent with this view of their origin. They
unexpectedly gave a uniform period. No trace of the retardation poleward
from the equator, shown by the spots, could be detected in their
movements. But the experiment was obviously inconclusive;[457] and M.
Stratonoff's[458] repetition of it with ampler materials gave a full
assurance that faculæ rotate like spots in periods lengthening as
latitude augments.
The ideas of M. Faye were, on two fundamental points, contradicated by
the Kew investigators. He held spots to be regions of _uprush_ and of
heightened temperature; they believed their obscurity to be due to a
_downrush_ of comparatively cool vapours. Now M. Chacornac, observing,
at Ville-Urbanne, March 6, 1865, saw floods of photospheric matter
visibly precipitating themselves into the abyss opened by a great spot,
and carrying with them small neighbouring maculæ.[459] Similar instances
were repeatedly noted by Father Secchi, who considered the existence of
a kind of _suction_ in spots to be quite beyond question.[460] The
tendency in their vicinity, to put it otherwise, is _centripetal_, not
_centrifugal_; and this alone seems to negative the supposition of a
central uprush.
A fresh witness was by this time at hand. The application of the
spectroscope to the direct examination of the sun's surface dates from
March 4, 1866, when Sir Norman Lockyer (to give him his present title)
undertook an inquiry into the cause of the darkening in spots.[461] It
was made possible by the simple device of throwing upon the slit of the
spectroscope an _image_ of the sun, any part of which could be subjected
to special scrutiny, instead of, as had hitherto been done, admitting
rays from every portion of his surface indiscriminately. The answer to
the inquiry was prompt and unmistakable, and was again, in this case,
adverse to the French theorist's view. The obscurations in question were
found to be produced by no deficiency of emissive power, but by an
increase of absorptive action. The background of variegated light
remains unchanged, but more of it is stopped by the interposition of a
dense mass of relatively cool vapours. The spectrum of a sun-spot is
crossed by the same set of multitudinous dark lines, with some minor
differences, visible in the ordinary solar spectrum. We must then
conclude that the same vapours (speaking generally) which are dispersed
over the unbroken solar surface are accumulated in the umbral cavity,
the compression incident to such accumulation being betrayed by the
thickening of certain lines of absorption. But there is also a general
absorption, extending almost continuously from one end of the
spot-spectrum to the other. Using, however, a spectroscope of
exceptionally high dispersive power, Professor Young of Princeton, New
Jersey, succeeded in 1883 in "resolving" the supposed continuous
obscurity of spot-spectra into a countless multitude of fine dark lines
set very close together.[462] Their structure was seen still more
perfectly, about five years later, by M. Dunér,[463] Director of the
Upsala Observatory, who traced besides some shadowy vestiges of the
crowded doublets and triplets forming the array, from the spots on to
the general solar surface. They cease to be separable in the blue part
of the spectrum; and the ultra-violet radiations of spots show nothing
distinctive.[464]
As to the movements of the constipated vapours forming spots, the
spectroscope is also competent to supply information. The principle of
the method by which it is procured will be explained farther on. Suffice
it here to say that the transport, at any considerable velocity, to or
from the eye of the gaseous material giving bright or dark lines, can be
measured by the displacement of such lines from their previously known
normal positions. In this way movements have been detected in or above
spots of enormous rapidity, ranging up to 320 _miles per second_. But
the result, so far, has been to negative the ascription to them of any
systematic direction. Uprushes and downrushes are doubtless, as Father
Cortie remarks,[465] "correlated phenomena in the production of a
sun-spot"; but neither seem to predominate as part of its regular
internal economy.
The same kind of spectroscopic evidence tells heavily against a theory
of sun-spots started by Faye in 1872. He had been foremost in pointing
out that the observations of Carrington and Spörer absolutely forbade
the supposition that any phenomenon at all resembling our trade-winds
exists in the sun. They showed, indeed, that beyond the parallels of 20°
there is a general tendency in spots to a slow poleward displacement,
while within that zone they incline to approach the equator; but their
"proper movements" gave no evidence of uniformly flowing currents in
latitude. The systematic drift of the photosphere is strictly a drift in
longitude; its direction is everywhere parallel to the equator. This
fact being once clearly recognised, the "solar tornado" hypothesis at
once fell to pieces; but M. Faye[466] perceived another source of
vorticose motion in the unequal rotating velocities of contiguous
portions of the photosphere. The "pores" with which the whole surface of
the sun is studded he took to be the smaller eddies resulting from these
inequalities; the spots to be such eddies developed into whirlpools. It
only needs to thrust a stick into a stream to produce the kind of effect
designated. And it happens that the differences of angular movement
adverted to attain a maximum just about the latitudes where spots are
most frequent and conspicuous.
There are, however, grave difficulties in identifying the two kinds of
phenomena. One (already mentioned) is the total absence of the regular
swirling motion--in a direction contrary to that of the hands of a watch
north of the solar equator, in the opposite sense south of it--which
should impress itself upon every lineament of a sun-spot if the cause
assigned were a primary producing, and not merely (as it possibly may
be) a secondary determining one. The other, pointed out by Young,[467]
is that the cause is inadequate to the effect. The difference of
movement, or _relative drift_, supposed to occasion such prodigious
disturbances, amounts, at the utmost, for two portions of the
photosphere 123 miles apart, to about five yards a minute. Thus the
friction of contiguous sections must be quite insignificant.
A view better justified by observation was urged by Secchi in and after
the year 1872, and was presented in an improved form by Professor Young
in his excellent little book on _The Sun_, published in 1882.[468] Spots
are manifestly associated with violent eruptive action, giving rise to
the faculæ and prominences which usually garnish their borders. It is
accordingly contended that upon the withdrawal of matter from below by
the flinging up of a prominence must ensue a sinking-in of the surface,
into which the partially cooled erupted vapours rush and settle,
producing just the kind of darkening by increased absorption told of by
the spectroscope. Round the edges of the cavity the rupture of the
photospheric shell will form lines of weakness provocative of further
eruptions, which will, in their turn, deepen and enlarge the cavity. The
phenomenon thus tends to perpetuate itself, until equilibrium is at last
restored by internal processes. A sun-spot might then be described as an
inverted terrestrial volcano, in which the outbursts of heated matter
take place on the borders instead of at the centre of the crater, while
the cooled products gather in the centre instead of at the borders.
But on the earth, the solid crust forcibly represses the steam gathering
beneath until it has accumulated strength for an explosion, while there
is no such restraining power that we know of in the sun. Zöllner,
indeed, adapted his theory of the solar constitution to the special
purpose of procuring it; yet with very partial success, since almost
every new fact has proved adverse to his assumptions. Volcanic action is
essentially spasmodic. It implies habitual constraint varied by
temporary outbreaks, inconceivable in a gaseous globe, such as we
believe the sun to be.
If the "volcanic hypothesis" represented the truth, no spot could
possibly appear without a precedent eruption. The real order of the
phenomenon, however, is exceedingly difficult to ascertain; nor is it
perhaps invariable. Although, in most cases, the "opening" shows first,
that may be simply because it is more easily seen. According to Father
Sidgreaves,[469] the disturbance has then already passed the incipient
stage. He considers it indeed "highly probable that the preparatory sign
of a new spot is always a small, bright patch of facula."
This sequence, if established, would be fatal to Lockyer's theory of
sun-spots, communicated to the Royal Society, May 6, 1886,[470] and
further developed some months later in his work on _The Chemistry of the
Sun_. Spots are represented in it as incidental to a vast system of
solar atmospheric circulation, starting with the polar out- and up-flows
indicated by observations during some total eclipses, and eventuating in
the plunge downward from great heights upon the photosphere of
prodigious masses of condensed materials. From these falls result,
primarily, spots; secondarily, through the answering uprushes in which
chemical and mechanical forces co-operate, their girdles of
flame-prominences. The evidence is, however, slight that such a
circulatory flow as would be needed to maintain this supposed cycle of
occurrences really prevails in the sun's atmosphere; and a similar
objection applies to an "anticyclonic theory" (so to designate it)
elaborated by Egon von Oppolzer in 1893.[471] August Schmidt's optical
rationale of solar phenomena[472] was, on the other hand, a complete
novelty, both in principle and development. Attractive to speculators
from its recondite nature and far-reaching scope, it by no means
commended itself to practical observers, intolerant of finding the all
but palpable realities of their daily experience dealt with as illusory
products of "circular refraction."
A singular circumstance has now to be recounted. On the 1st of
September, 1859, while Carrington was engaged in his daily work of
measuring the positions of sun-spots, he was startled by the sudden
appearance of two patches of peculiarly intense light within the area of
the largest group visible. His first idea was that a ray of unmitigated
sunshine had penetrated the screen employed to reduce the brilliancy of
the image; but, having quickly convinced himself to the contrary, he ran
to summon an additional witness of an unmistakably remarkable
occurrence. On his return he was disappointed to find the strange
luminous outburst already on the wane; shortly afterwards the last trace
vanished. Its entire duration was five minutes--from 11.18 to 11.23
A.M., Greenwich time; and during those five minutes it had traversed a
space estimated at 35,000 miles! No perceptible change took place in the
details of the group of spots visited by this transitory conflagration,
which, it was accordingly inferred, took place at a considerable height
above it.[473]
Carrington's account was precisely confirmed by an observation made at
Highgate. Mr. R. Hodgson described the appearance seen by him as that
"of a very brilliant star of light, much brighter than the sun's
surface, most dazzling to the protected eye, illuminating the upper
edges of the adjacent spots and streaks, not unlike in effect the edging
of the clouds at sunset."[474]
This unique phenomenon seemed as if specially designed to accentuate
the inference of a sympathetic relation between the earth and the sun.
From the 28th of August to the 4th of September, 1859, a magnetic
storm of unparalleled intensity, extent, and duration, was in progress
over the entire globe. Telegraphic communication was everywhere
interrupted--except, indeed, that it was, in some cases, found
practicable to work the lines _without batteries_, by the agency of
the earth-currents alone:[475] sparks issued from the wires; gorgeous
auroræ draped the skies in solemn crimson over both hemispheres, and
even within the tropics; the magnetic needle lost all trace of
continuity in its movements, and darted to and fro as if stricken with
inexplicable panic. The coincidence was drawn even closer. _At the
very instant_[476] of the solar outburst witnessed by Carrington and
Hodgson, the photographic apparatus at Kew registered a marked
disturbance of all the three magnetic elements; while, shortly after
the ensuing midnight, the electric agitation culminated, thrilling the
earth with subtle vibrations, and lighting up the atmosphere from pole
to pole with the coruscating splendours which, perhaps, dimly recall
the times when our ancient planet itself shone as a star.
Here then, at least, the sun was--in Professor Balfour Stewart's
phrase--"taken in the act"[477] of stirring up terrestrial commotions.
Nor have instances since been wanting of an indubitable connection
between outbreaks of individual spots and magnetic disturbances. Four
such were registered in 1882; and symptoms of the same kind, including
the beautiful "Rose Aurora," marked the progress across the sun of the
enormous spot-group of February, 1892--the largest ever recorded at
Greenwich. This extraordinary formation, which covered about 1/300 of
the sun's disc, survived through five complete rotations.[478] It was
remarkable for a persistent drift in latitude, its place altering
progressively from 17° to 30° south of the solar equator.
Again, the central passage of an enormous spot on September 9, 1898,
synchronised with a sharp magnetic disturbance and brilliant
aurora;[479] and the coincidence was substantially repeated in March,
1899,[480] when it was emphasised by the prevalent cosmic calm. The
theory of the connection is indeed far from clear. Lord Kelvin, in
1892,[481] pronounced against the possibility of any direct magnetic
action by the sun upon the earth, on the ground of its involving an
extravagant output of energy; but the fact is unquestionable that--in
Professor Bigelow's words--"abnormal agitations affect the sun and the
earth as a whole and at the same time."[482]
The nearer approach to the event of September 1, 1859, was
photographically observed by Professor George E. Hale at Chicago, July
15, 1892.[483] An active spot in the southern hemisphere was the scene
of this curiously sudden manifestation. During an interval of 12m.
between two successive exposures, a bridge of dazzling light was found
to have spanned the boundary-line dividing the twin-nuclei of the spot;
and these, after another 27m., were themselves almost obliterated by an
overflow of far-spreading brilliancy. Yet two hours later, no trace of
the outburst remained, the spot and its attendant faculæ remaining just
as they had been previously to its occurrence. Unlike that seen by
Carrington, it was accompanied by no exceptional magnetic phenomena,
although a "storm" set in next day.[484] Possibly a terrestrial analogue
to the former might be discovered in the "auroral beam" which traversed
the heavens during a vivid display of polar lights, November 17, 1882,
and shared, there is every reason to believe, their electrical origin
and character.[485]
Meantime M. Rudolf Wolf, transferred to the direction of the Zürich
Observatory, where he died, December 6, 1893, had relaxed none of his
zeal in the investigation of sun-spot periodicity. A laborious revision
of the entire subject with the aid of fresh materials led him, in
1859,[486] to the conclusion that while the _mean_ period differed
little from that arrived at in 1852 of 11.11 years, very considerable
fluctuations on either side of that mean were rather the rule than the
exception. Indeed, the phrase "sun-spot period" must be understood as
fitting very loosely the great fact it is taken to represent; so
loosely, that the interval between two maxima may rise to sixteen and a
half or sink below seven and a half years.[487] In 1861[488] Wolf
showed, and the remark was fully confirmed at Kew, that the shortest
periods brought the most acute crises, and _vice versâ_; as if for each
wave of disturbance a strictly equal amount of energy were available,
which might spend itself lavishly and rapidly, or slowly and
parsimoniously, but could in no case be exceeded. The further inclusion
of recurring solar commotions within a cycle of fifty-five and a half
years was simultaneously pointed out; and Hermann Fritz showed soon
afterwards that the aurora borealis is subject to an identical double
periodicity.[489] The same inquirer has more recently detected both for
auroræ and sun-spots a "secular period" of 222 years,[490] and the Kew
observations indicate for the latter, oscillations accomplished within
twenty-six and twenty-four days,[491] depending, most likely, upon the
rotation of the sun. This is certainly reflected in magnetic, and
perhaps in auroral periodicity. The more closely, in fact,
spot-fluctuations are looked into, the more complex they prove. Maxima
of one order are superposed upon, or in part neutralised by, maxima of
another order;[492] originating causes are masked by modifying causes;
the larger waves of the commotion are indented with minor undulations,
and these again crisped with tiny ripples, while the whole rises and
falls with the swell of the great secular wave, scarcely perceptible in
its progress because so vast in scale.
The idea that solar maculation depends in some way upon the position of
the planets occurred to Galileo in 1612.[493] It has been industriously
sifted by a whole bevy of modern solar physicists. Wolf in 1859[494]
found reason to believe that the eleven-year curve is determined by the
action of Jupiter, modified by that of Saturn, and diversified by
influences proceeding from the earth and Venus. Its tempting approach to
agreement with Jupiter's period of revolution round the sun, indeed,
irresistibly suggested a causal connection; yet it does not seem that
the most skilful "coaxing" of figures can bring about a fundamental
harmony. Carrington pointed out in 1863, that while, during _eight
successive periods_, from 1770 downwards, there were approximate
coincidences between Jupiter's aphelion passages and sun-spot maxima,
the relation had been almost exactly reversed in the two periods
preceding that date;[495] and Wolf himself finally concluded that the
Jovian origin must be abandoned.[496] M. Duponchel's[497] prediction,
nevertheless, of an abnormal retardation of the maximum due in 1881
through certain peculiarities in the positions of Uranus and Neptune
about the time it fell due, was partially verified by the event, since,
after an abortive phase of agitation in April, 1882, the final outburst
was postponed to January, 1894. The interval was thus 13.5 instead of
11.1 years; and it is noticeable that the delay affected chiefly the
southern hemisphere. Alternations of activity in the solar hemispheres
were indeed a marked feature of the maximum of 1884, which, in M. Faye's
view,[498] derived thence its indecisive character, while sharp, strong
crises arise with the simultaneous advance of agitation north and south
of the solar equator. The curve of magnetic disturbance followed with
its usual strict fidelity the anomalous fluctuations of the sun-spot
curve. The ensuing minimum occurred early in 1889, and was succeeded in
1894 by a maximum slightly less feeble than its predecessor.[499]
It cannot be said that much progress has been made towards the
disclosure of the cause, or causes, of the sun-spot cycle. No external
influence adequate to the effect has, at any rate, yet been pointed out.
Most thinkers on this difficult subject provide a quasi-explanation of
the periodicity in question through certain assumed vicissitudes
affecting internal processes;[500] Sir Norman Lockyer and E. von
Oppolzer reach the same end by establishing self-compensatory
fluctuations in the solar atmospheric circulation; Dr. Schuster resorts
to changes in the electrical conductivity of space near the sun.[501] In
all these theories, however, the course of transition is arbitrarily
arranged to suit a period, which imposes itself as a fact peremptorily
claiming admittance, while obstinately defying explanation.
The question so much discussed, as to the influence of sun-spots on
weather, does not admit of a satisfactory answer. The facts of
meteorology are too complex for easy or certain classification. Effects
owning dependence on one cause often wear the livery of another; the
meaning of observed particulars may be inverted by situation; and yet it
is only by the collection and collocation of particulars that we can
hope to reach any general law. There is, however, a good deal of
evidence to support the opinion--the grounds for which were primarily
derived from the labours of Dr. Meldrum at Mauritius--that increased
rainfall and atmospheric agitation attend spot-maxima; while Herschel's
conjecture of a more copious emission of light and heat about the same
epochs has recently obtained some countenance from Savélieff's measures
showing a gain in the strength of the sun's radiation _pari passu_ with
increase in the number of spots visible on his surface.[502]
The examination of what we may call the _texture_ of the sun's surface
derived new interest from a remarkable announcement made by Mr. James
Nasmyth in 1862.[503] He had made (as he supposed) the discovery that
the entire luminous stratum of the sun is composed of a multitude of
elongated shining objects on a darker background, shaped much like
willow-leaves, of vast size, crossing each other in all possible
directions, and possessed of unceasing relative motions. A lively
controversy ensued. In England and abroad the most powerful telescopes
were directed to a scrutiny encompassed with varied difficulties. Mr.
Dawes was especially emphatic in declaring that Nasmyth's
"willow-leaves" were nothing more than the "nodules" of Sir William
Herschel seen under a misleading aspect of uniformity; and there is
little doubt that he was right. It is, nevertheless, admitted that
something of the kind may be seen in the penumbræ and "bridges" of
spots, presenting an appearance compared by Dawes himself in 1852 to
that of a piece of coarse straw-thatching left untrimmed at the
edges.[504]
The term "granulated," suggested by Dawes in 1864,[505] best describes
the mottled aspect of the solar disc as shown by modern telescopes and
cameras. The grains, or rather the "floccules," with which it is thickly
strewn, have been resolved by Langley, under exceptionally favourable
conditions, into "granules" not above 100 miles in diameter; and from
these relatively minute elements, composing, jointly, about one-fifth of
the visible photosphere,[506] he estimates that three-quarters of the
entire light of the sun are derived.[507] Janssen agrees, so far as to
say that if the whole surface were as bright as its brightest parts, its
luminous emission would be ten to twenty times greater than it actually
is.[508]
The rapid changes in the forms of these solar cloud-summits are
beautifully shown in the marvellous photographs taken by Janssen at
Meudon, with exposures reduced at times to 1/100000 of a second! By
their means, also, the curious phenomenon known as the _réseau
photosphérique_ has been made evident.[509] This consists in the
diffusion over the entire disc of fleeting blurred patches, separated by
a reticulation of sharply-outlined and regularly-arranged granules. The
imperfect definition in the smudged areas may be due to agitations in
the solar or terrestrial atmosphere, unless it be--as Dr. Schemer thinks
possible[510]--merely a photographic effect. M. Janssen considers that
the photospheric cloudlets change their shape and character with the
progress of the sun-spot period;[511] but this is as yet uncertain.
The "grains," or more brilliant parts of the photosphere, are now
generally held to represent the upper termination of ascending and
condensing currents, while the darker interstices (Herschel's "pores")
mark the positions of descending cooler ones. In the penumbræ of spots,
the glowing streams rushing up from the tremendous sub-solar furnace are
bent sideways by the powerful indraught, so as to change their vertical
for a nearly horizontal motion, and are thus taken, as it were, in flank
by the eye, instead of being seen end-on in mamelon-form. This gives a
plausible explanation of the channelled structure of penumbræ which
suggested the comparison to a rude thatch. Accepting this theory as in
the main correct, we perceive that the very same circulatory process
which, in its spasms of activity, gives rise to spots, produces in its
regular course the singular "marbled" appearance, for the recording of
which we are no longer at the mercy of the fugitive or delusive
impressions of the human retina. And precisely this circulatory process
it is which gives to our great luminary its permance as a _sun_, or
warming and illuminating body.
FOOTNOTES:
[Footnote 405: _Mem. R. A. S._, vol. xxi., p. 157.]
[Footnote 406: _Ibid._, p. 160.]
[Footnote 407: _Month. Not._, vol. xxi., p. 144.]
[Footnote 408: _Le Soleil_, t. i., pp. 87-90 (2nd ed., 1871).]
[Footnote 409: See _ante_, p. 58.]
[Footnote 410: _Observations at Redhill (1863)_, Introduction.]
[Footnote 411: _Month. Not._, vol. xxxvi., p. 142.]
[Footnote 412: _Cape Observations_, p. 435, _note_.]
[Footnote 413: _Month. Not._, vol. x., p. 158.]
[Footnote 414: _Rosa Ursina_, lib. iii., p. 348.]
[Footnote 415: _Observations at Redhill_, p. 8.]
[Footnote 416: _Op._, t. iii., p. 402.]
[Footnote 417: _Rosa Ursina_, lib. iv., p. 601. Both Galileo and
Scheiner spoke of the _apparent_ or "synodical" period, which is about
one and a third days longer than the _true_ or "sidereal" one. The
difference is caused by the revolution of the earth in its orbit in the
same direction with the sun's rotation on its axis.]
[Footnote 418: _Rosa Ursina_, lib. iii., p. 260.]
[Footnote 419: Faye, _Comptes Rendus_, t. lx., p. 818.]
[Footnote 420: _Ibid._, t. xii., p. 648.]
[Footnote 421: _Proc. Am. Ass. Adv. of Science_, 1885, p. 85.]
[Footnote 422: _Observations at Redhill_, p. 221.]
[Footnote 423: _Am. Jour. of Science_, vol. xi., p. 169.]
[Footnote 424: _Month. Not._, vol. xix., p. 1.]
[Footnote 425: _Vierteljahrsschrift der Naturfors. Gesellschaft_
(Zürich), 1859, p. 252.]
[Footnote 426: Lockyer, _Chemistry of the Sun_, p. 428.]
[Footnote 427: Maunder, _Knowledge_, vol. xv., p. 130.]
[Footnote 428: _Month. Mon._, vol. l., p. 251.]
[Footnote 429: Maunder, _Knowledge_, vol. xvii., p. 173.]
[Footnote 430: _Astr. Nach._, No. 1,315.]
[Footnote 431: As late as 1866 an elaborate treatise in its support was
written by F. Coyteux, entitled _Qu'est-ce que le Soleil? Peut-il être
habité?_ and answering the question in the affirmative.]
[Footnote 432: The subsequent researches of Plücker, Frankland, Wüllner,
and others, showed that gases strongly compressed give an absolutely
unbroken spectrum.]
[Footnote 433: _Comptes Rendus_, t. lx., pp. 89, 138.]
[Footnote 434: _Ibid._, t. c., p. 595.]
[Footnote 435: _Bull. Meteor. dell Osservatorio dell Coll. Rom._, Jan.
1, 1864, p. 4.]
[Footnote 436: _Quart. Jour. of Science_, vol. i., p. 222.]
[Footnote 437: _Ann. de Chim. et de Phys._, t. xxii., p. 127.]
[Footnote 438: _Phil. Trans._, vol. clix., p. 575.]
[Footnote 439: _Les Mondes_, Dec. 22, 1864, p. 707.]
[Footnote 440: _Comptes Rendus_, t. lx., p. 147.]
[Footnote 441: _Proc. Roy. Society_, vol. xvi., p. 29.]
[Footnote 442: _Recherches sur le Spectre Solaire_, p. 38.]
[Footnote 443: _Am. Jour. of Science_, 1881, vol. xxi., p. 41. Hastings
stipulated only for some member of the triad, carbon, silicon, and
boron.]
[Footnote 444: Ranyard, _Knowledge_, vol. xvi., p. 190.]
[Footnote 445: Young, _The Sun_, p. 337, ed. 1897.]
[Footnote 446: H. Draper, _Quart. Journ. of Sc._, vol. i., p. 381; also
_Phil. Mag._, vol. xvii., 1840, p. 222.]
[Footnote 447: Reproduced in Arago's _Popular Astronomy_, plate xii.,
vol. 1.]
[Footnote 448: _Report Brit. Ass._, 1859, p. 148.]
[Footnote 449: _Phil. Trans._, vol. clii., p. 407.]
[Footnote 450: _Researches in Solar Physics_, part i., p. 20.]
[Footnote 451: Both the phrase and the method were suggested by Faye,
who estimated the average depth of the luminous sheath of spots at 2,160
miles. _Comptes Rendus_, t. lxi., p. 1082; t. xcvi., p. 356.]
[Footnote 452: _Month. Not._, vol. lv., p. 74.]
[Footnote 453: Sidgreaves, _Ibid._, p. 282; Cortie, _Ibid._, vol.
lviii., p. 91.]
[Footnote 454: Explained by East as refraction-effects. _Jour. Brit.
Astr. Ass._, vol. viii., p. 187.]
[Footnote 455: _Proc. Roy. Soc._, vol. xiv., p. 39.]
[Footnote 456: _Potsdam Publicationen_, No. 18; _Astr. Nach._, Nos.
3,000, 3,287.]
[Footnote 457: Faye, _Comptes Rendus_, t. cxi., p. 77; Bélopolsky,
_Astr. Nach._, No. 2,991.]
[Footnote 458: _Ibid._, Nos. 3,275, 3,344.]
[Footnote 459: Lockyer, _Contributions to Solar Physics_, p. 70.]
[Footnote 460: _Le Soleil_, p. 87.]
[Footnote 461: _Proc. Roy. Soc._, vol. xv., p. 256.]
[Footnote 462: _Phil. Mag._, vol. xvi., p. 460.]
[Footnote 463: _Recherches sur la Rotation du Soleil_, p. 12.]
[Footnote 464: Hale, _Astr. and Astrophysics_, vol. xi., p. 814.]
[Footnote 465: _Jour. Brit. Astr. Ass._, vol. i., p. 177.]
[Footnote 466: _Comptes Rendus_, t. lxxv., p. 1664; _Revue
Scientifique_, t. v., p. 359 (1883). Mr. Herbert Spencer had already (in
_The Reader_, Feb. 25, 1865) put forward an opinion that spots were of
the nature of "cyclonic clouds."]
[Footnote 467: _The Sun_, p. 174. For Faye's answer to the objection,
see _Comptes Rendus_, t. xcv., p. 1310.]
[Footnote 468: A revised edition appeared in 1897.]
[Footnote 469: _Astr. and Astrophysics_, vol. xii., p. 832.]
[Footnote 470: _Proc. Roy. Soc._, No. 244.]
[Footnote 471: _Astr. Nach._, No. 3,146; _Astr. and Astrophysics_, vol.
xii., pp. 419, 736.]
[Footnote 472: _Sirius_, Sept., 1893; _ibid._, vol. xxiii., p. 97;
_Astrophy. Jour._, vol. i., p. 112 (Wilczynski), p. 178 (Keeler); vol.
ii., p. 73 (Hale).]
[Footnote 473: _Month. Not._, vol. xx., p. 13.]
[Footnote 474: _Ibid._, p. 15.]
[Footnote 475: _Am. Jour._, vol. xxix. (2nd series), pp. 94, 95.]
[Footnote 476: The magnetic disturbance took place at 11.15 A.M., three
minutes before the solar blaze compelled the attention of Carrington.]
[Footnote 477: _Phil. Trans._, vol. cli., p. 428.]
[Footnote 478: Maunder, _Journal Brit. Astr. Ass._, vol. ii., p. 386;
Miss E. Brown, _Ibid._, p. 210; Month. Not., vol. lii., p. 354.]
[Footnote 479: _Observatory_, vol. xxi., p. 387; Maunder, _Knowledge_,
vol. xxi., p. 228; Fényi, _Astroph. Jour._, vol. x., p. 333.]
[Footnote 480: _Ibid._, p. 336; W. Anderson, Observatory, vol. xxii., p.
196.]
[Footnote 481: _Proc. Roy. Society_, vol. lii., p. 307; Rev. W.
Sidgreaves, _Mem. R. A. S._, vol. liv., p. 85.]
[Footnote 482: _Report on Solar and Terrestrial Magnetism_, Washington,
1898, p. 27.]
[Footnote 483: _Astr. and Astrophysics_, vol. xi., p. 611.]
[Footnote 484: _Ibid._, p. 819 (Sidgreaves).]
[Footnote 485: See J. Rand Capron, _Phil. Mag._, vol. xv., p. 318.]
[Footnote 486: _Mittheilungen über die Sonnenflecken_, No. ix.,
_Vierteljahrsschrift der Naturforschenden Gesellschaft in Zürich_,
Jahrgang 4.]
[Footnote 487: _Mitth._, No. lii., p. 58 (1881).]
[Footnote 488: _Ibid._, No. xii., p. 192. Baxendell, of Manchester,
reached independently a similar conclusion. See _Month. Not._, vol.
xxi., p. 141.]
[Footnote 489: Wolf, _Mitth._, No. xv., p. 107, etc. Olmsted, following
Hansteen, had already, in 1856, sought to establish an auroral period of
sixty-five years. _Smithsonian Contributions_, vol. viii., p. 37.]
[Footnote 490: Hahn, _Ueber die Reziehungen der Sonnenfleckenperiode zu
meteorologischen Erscheinungen_, p. 99 (1877).]
[Footnote 491: _Report Brit. Ass._, 1881, p. 518; 1883, p. 418.]
[Footnote 492: The Rev. A. Cortie (_Month. Not._, vol. lx., p. 538)
detects the influence of a short subsidiary cycle, Dr. W. J. S. Lockyer
that of a thirty-five year period (_Nature_, June 20, 1901). Professor
Newcomb (_Astroph. Jour._, vol. xiii., p. 11) considers that solar
activity oscillates uniformly in 11.13 years, with superposed periodic
variations.]
[Footnote 493: _Opere_, t. iii., p. 412.]
[Footnote 494: _Mitth._, Nos. vii. and xviii.]
[Footnote 495: _Observations at Redhill_, p. 248.]
[Footnote 496: _Comptes Rendus_, t. xcv., p. 1249.]
[Footnote 497: _Ibid._, t. xciii., p. 827; t. xcvi., p. 1418.]
[Footnote 498: _Ibid._, t. c, p. 593.]
[Footnote 499: Ellis, _Proc. Roy. Society_, vol. lxiii., p. 70.]
[Footnote 500: Schultz, _Astr. Nach._, Nos. 2,817-18, 2,847-8; Wilsing,
_Ibid._, No. 3,039; Bélopolsky, _Ibid._, No. 2,722.]
[Footnote 501: _Report Brit. Ass._, 1892, p. 635.]
[Footnote 502: A. W. Augur, _Astroph. Jour._, vol. xiii., p. 346.]
[Footnote 503: _Report Brit. Ass._, 1862, p. 16 (pt. ii.).]
[Footnote 504: _Mem. R. A. S._, vol. xxi., p. 161.]
[Footnote 505: _Month. Not._, vol. xxiv., p. 162.]
[Footnote 506: _Am. Jour. of Science_, vol. vii., 1874, p. 92.]
[Footnote 507: Young, _The Sun_, p. 103.]
[Footnote 508: _Ann. Bur. Long._, 1879, p. 679.]
[Footnote 509: _Ibid._, 1878, p. 689.]
[Footnote 510: _Himmelsphotographie_, p. 273.]
[Footnote 511: Ranyard, _Knowledge_, vols. xiv., p. 14, xvi., p. 189;
see also the accompanying photographs.]