John Almon.

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also exhibited a pictorial history of each of these groups in his
published volume, in which the spots are represented, though
on a smaller scale than in his original drawings, all that is neces-
sary on our part is to accompany any remark we may make re-
garaing one of Carrington's groups with the number of that
group as given by him.

Next with regard to the Kew pictures. Beginning with the
first picture in 1858, it is our intention to numl^r each group of
spots in the same manner as Carrington, calline the first No. 1,
and so on upwards to the present date. It is also our intention
ultimately to publish carefully copied representations of each of
the Kew groups ; but these are not yet ready. We think it
well, however, to give at once the numbers of the groups,
coupled with the d^tes at which each group was first seen, and
also to make use of these numbers in our present paper in an-
ticipation of the forthcoming pictorial representations, which,
when they appear, will enable our readers to judge for them-
selves of the truth of our remarks.

[The table and some comparisons of the Kew observations
with those of Schwabe are omitted.]

§ V. Two classes of investigations.

16. Our investigations mav be divided into two classes:

(1.) Those in which remarks are made regarding the behavior
and appearance of spots and faculse, and generalizations deduced
theretrom, which do not involve accurate measurements.

(2.) There are, however, certain results in order to obtain
which it is necessary to make use of accurate measurements of
the position of spots: such are those from which Carrington has
deduced the proper motion of spots on the sun's surface. •Prob-
ably for this class of observations no better method can be
adopted than that so ably pursued by him ; but since we have
materials at our disposal embracing accurate photographic de-
lineations, we are perhaps called upon to attempt corrections
which he has not applied.

19. Correction for solar atmosphere. — The most important of
these is the correction due to the refraction of the solar atmos-
phere, which Carrington has indicated, but without appljring it^
m his large volume. There are evident proo& of the existence
of such aa atmosphere ; for

(1.) In the Kew photographs the central portion of the disk
uniformly indicates a greater luminosity than the borders, as if
the rays at the borders had to pass through a large extent of



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R$$earche$ on Solar Pky$%es. 185

atmosphere. It is worthy of remark that the temperature of
this atmosphere must be lower than that of the photosphere ;
otherwise the absorptioa which it occasions woula be counter-
balanced bj its radiation.

(2.) The beautirul discovery of Eirchhoff leads to the same
conclusion, since, in order to account for the dark lines of the
Bolar spectrum, it is necessary to suppose the existence of a solar
atmosphere of a lower temperature than the source of light

(8.) The red flames which are visible during a total eclipse,
and which have been proved to belong to the sun (Art. 9), indi*
cate the existence of a solar atmosphere extending in some in*
stances as far as 72,000 miles above the photosphere. This is
confirmed by the nature of the light which these flames emit
Mr. De la Bue has found that this light is very rich in actinic
rays, so much so-that he was able to photograph at least one
protuberance which was not visible to the eye. Now it is pre*
cisely this description of light which characterizes the electric
discharge in which gaseous matter appears in a highly heated
state.

20. Let us now endeavor to show the nature of those correo*
lions which are rendered necessary by solar refraction.

(1.) A solar atmosphere will make the sun's photosphere to
appear larger than it really is; but the angular distance oetween
two points, each near the center of the visible disk, will not be
appreciably altered. This will introduce a slight error into the
calculated position of any point, since in such a calculation we
make use of the sun's apparent angular diameter, which is
greater than his true diameter.

(2.) Apart from this, an error will be introduced into the cal-
culation of the solar latitude and longitude of a point, this error
depending upon its position in the visible disk, and being greater
for those points which are at a distance from the center.

§ VI. QueBiioM to he answered in the present paper,

21. In the present paper we shall attempt to answer the fol-
lowing questions : —

(I.) Is the umbra of a spot nearer the sun's center than its
penumbra? or, in other words, is it at a lower level?

(IL) Is the photosphere of our luminary to be viewed as com-
posed of heavy solid, or heavy liquid matter? or is it rather of
the nature of a cloud ? A short explanation will render evident
the meaning of this c[uestion. There are two types, either of
which we may conceive as representing the solar photosphere :
we may, in the first place, suppose it to be a solid or liquid
plane more or less uneven, with a heavy atmosphere above it
This atmosphere may be comoosed either of quite different ma-
terials from those of the liquia plane, or it may contain some of
the materials of the plane in a state of vapor. Our own ocean is



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Ruearcheg on Solar Phy$ic$^



an example of this type, the air above it beinff composed chiefly
of matenals different from those of the ocean, but containing also
aqueoQS vapor. On the other hand, we may imagine the sun's
photosphere to resemble a cloud, the characteristic of which is
solid or liquid particles of a greater or less size existing in a
gaseous atmosphere, composed to a greater or less extent of the
materials of the cloud. These points must be determined bj
observation alone. We must ask if the appearances presentel
by the son's photosphere lead to the conclusion that it is an un-
even plane of heavy liquid or solid matter, or do they induce
UB to imagine that it is rather of the nature of a cloud ?

(IIL) Is a spot^ including both umbra and penumbra, a pbe-
Domenon which takes place beneath the level of the sun's pho-
tosphere or above it?

22. In the first place, therefore, and to answer the first ques-
tion, let us see what will happen if the umbm of a spot be nearer
the sun's center than the penumbra?

If the umbra be lower than the penumbra, when a spot passes
over the sun's disk, the umbra will always appear to encroach
upon that side of the penumbra which is directed toward the
visual center of the sun's disk; and this effect will be lessened
through the refraction caused bv a solar atmosphere, bat we
cannot conceive that it will be wholly obliterated. If therefore
the umbra is appreciably at a lower level than the penumbra,
we are entitled to look for an apparent encroachment of the
former upon the latter on that side which is nearest the visual
oenter of the disk. This in fact was the phenomenon whidi
Wilson observed, and which led him to the belief that the ambra
was nearer the sun's center than the penumbra.

23. In the following six sub-tables the effect of foreshortening
is estimated in the direction from left to ri^ht, this being the
direction in which spots advance across the visible disk by rota-
tion ; and for this purpose the whole surface of the sun has been
divided into six portions, comprising 80° each.

[These sub-tables are omitted, the general results fixmi them
being given in the following summaries.]

Rendi of Table II«. Showing the effict of foreshortening in the direc-
tian left and right of the central line,

ff. Giving the mean ratios between the tvo sides of the penumbra.



Left of Mntral line. ]


WitliladO'rnKiitofc
link


BotweoD 90* and 60° from

loft llffib.


Within 3^ from centnl
line.


No. or

■pots


ThA peottiBbra on tho

r^Msideof aipot

boiof oqMl to mity,

that on tho to/Kit

oqoolto


No. of
■pote


Tho ponurobra oo tho

right lide of a upot

heiofi equal to vaity,

that 00 the left it

oqoalto


Naof
■pots

obieired.


The poDumbra on tfae

rirht fide of a ipoC

boiojr eqoal to oaity,

that on the t^t U

eqaalto


UO


1-8


119


1-4


88


1-2



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187



Rmdi of Tablv Ila. **ooptioiMd.



AfAlofcsatnillliM. |


Wttta 30* ftom Mitral

liM.


BetWMA ao* aod 60* froB

tifbt llmbw


Within 30" from right
limb.


H


TIm iMDDinbni oo the

IdlmdaoTafpot

bcinf equal to unity,

tWootheW^A/li

•qnlto


W*of


Til* penumbra on the

Un side nf « ipot

being equal to uoity,

tbct OB the right it

equal to


Naof

ipou

obaerred.


The penumbra on the

fe/feideofatpot

being equal to unity,

that OB the Hf A/ It

equal to


91


1-2


77


1-8


100


1-6



&. Qitimg the percentage of ctues, cut of 580 oUervatume in all, which are in cofi-
frnmify frith the aeettmptiont that the wnhra ie nearer to the center of the tun than
ihepemmtbra, emd of came fMek are againtt iL



L^ofoeBUalline.


Witiiinao-fioaileft
limbi


Between '<iXf and &f from
left limb.


Within 30* ftom central
Une.


Fei;


Aguinet.


For.


Agaiurt.


For.


Agalnn. 1


Re


Pteeeat


No.


Per cent.


No.


PtoreoDt


Na


Per cent.


No.

66


Percent


Ne.


Pereeut.


119 1 92-8


10


7-7


99


90-0


11


100


861


9


18-9


Right of central line.


llntUnaO-AumcMitnl

line.


Beitweeaa0*and6(rfc«m
right limb.


Withia 80* from right
limb.


- Fee.


Afulnn.


For. AguiHt


For.


Agaiot ]


Ka


Pereent
74-2


No.
16


Percent


No.


Per cent


Na


Poreent


No. Percent


No.


Percent
14*6





«6-8


66


79-7


14


20-8


82 86-4


14



The whole number of cases observed is 605 ; excluding here-
&om 73, where the penumbra is equal on both sides, there re-
naio 530, of which 456, or 8604 per cent are for, and 74, or
13'96 per cent are against the assumption.

Result of Table Hi,.
Skm^g the effect of foreehortening in the direction abcwe and below the JBquator.



Abore the equator.



ikt MMnCra of |

c«hipetbeiBg

i«qniteiiaity,tbe!



Ftor



Againat
aaaumptloa.



'>ff«ptitisequarN«.er| Per iNo. of] Per



h-m-l



eaM» I oent jCaaes.



42 I 87-6 I 6



12-6



Below the equator.



The upper part of
ibra <
. beiBff
equal to unity, the
lower part !• equal



the penumbra of
beii



the lien
eaeb ip



1*88



For



No. of



80



Per

coat

78-2



Agalnit



Naof P;



11 26-8



The whole number of cases considered is 89, of which 72, or
809 per cent, are for, and 17, or 191 per cent, against the as-
samption, that the umbra is nearer to the center of the sun than
the penumbra.

In table lib, since the relative disposition of umbra and pen-
tu&bra is estimated in directions parallel to circles of solar Ion*
9^de, only spots having a higher solar latitude have been con-
Bidered.

It will be seen that the results of tables ii« and lib are deci**
^7 ia &VW of Wilson's hypothesis.



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IBS Re$earche$ on Solar Pkysict,

24. Let U8 now endeavor to answer the second question—* "Is
the photosphere of our luminary to be viewed as composed of
heavy liquid or solid matter, or is it of the nature of a cloud?"
One characteristic of the sun's surface is the appearance (espe-
cially in connexion with spots) of facule, or patcnes of a bright-
ness greater than that of the photosphere immediately around
them, this difference in brightness being much more conspicuous
near the limb than near the center. One explanation of these
phenomena would be, that the luminous matter of the sun has
oeen thrown up to a great elevation in order to form faculad.
l^his would account for the greater comparative luminosity of
facuIflB near the border. For we have already mentioned (Art
19) that the absorbing effect of the solar atmosphere is very per-
ceptible near the border, where the light reaching us has to
travel through a greater thickness of atmosphere ; and hence, if
the luminous matter be thrown up to a great elevation, it will,
near the border, escape a great portion oi this atmosphere, and
will therefore appear relatively much brighter than the surface
around it. On the other hand, very little will be gained when
the matter is thrown up near the visual center, where we may
imagine the atmospheric absorption to be comparatively small.
The idea that faculse are portions of the photosphere raised
above the general surface, appears to be confirmed by stereo-
scopic pictures of spots obtained by Mr. De la Rue, where the
faculae appear as elevated ridges surrounding the spots. Accept-
ing this conclusion, we next remark that faculas often retain the
same appearance for several days together, as if their matter
were capable of remaining suspended for some tin^e.

Now if we suppose that such faculad represent the ordinary
luminous matter of the sun, the facts above recorded would ap-
pear to throw much light upoi^ the nature of the solar surface,
since we cannot imagine faculae to be the most elevated positions
of a liquid ocean, which has been pushed high up into the solar
atmosphere, or to be portions of matter projectea from such an
ocean. Such an hypothesis would appear to be inconsistent
with the fact that the faculas retain their appearance unchanged
for days together. At any rate we venture to think that such
an hypothesis would not readily be received, and that, according
to the rules which ought to guide our judgment in a case like
the present, it ought to be set aside if we can find a more plau-
sible explanation. Such an explanation would appear to con-
sist in supposing that faculae, and, indeed, the whole photosphere
of our luminary, are more of the nature of a cloud. A cloud
has been defined by Sir J. Herschel to consist of solid or liquid
matter, formed from the condensation of a vapor noi floating in
aequilibrio, but sinking in a gaseous medium of less specific
gravity than itself— sinking, however, with extreme slowness,



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189



owing to the minateneas of its particles, and consequent (rela*
tively) enormous resistance of the air. This illustrious savant
is disposed to think that the consideration of Cagniard de La-
tour's experiments on the vaporization of liquids under high
pressure would incline us to n^rd the solar facul» as large ag-
gregations of bond fide solid matter of a high degree of nxily,
and in masses like gigantic soot-flakes of any form and magni-
tude, which, when formed, settle down to such a level as corre-
sponds to their density when they rest in cequilibrio in a gaseous
fluid of their own specific gravity. We do not wish either to
accept or to reject this hypothesis, but would frame the follow-
ing statement, which also includes this view of the case: Solar
fijculm consist of solid or liquid bodies of a grectter or less magnitude,
either slowly sinking or suspended in asquHibrio in a gaseous medium.

26. In connection with this part of our subject it will be well
to investigate the relative position of spots and their acc9mpa-
nying faculae ; and this is done in the following table for all the
]lf ew pictures available for this purpose.

[Table in is omitted.]

Result of Table III.



Faenla ootlrtly or orattly
te/(orqx>t


Faenla entirplr or moatly
Hght of ipot.


Faenla all round or hetween
thespota.


No. of
eaM«.


Per cent of
the whole.


NaoT
Maec


Per oeot of
the whole.


No. of
eaiea.


Per eent of
the whole.


684


61-4


46


40


608


44*6



26. It appears from the result of table ill, that out of 1187
cases 684 have their faculse either entirely or mostly on the left,
while 608 have it nearly equally on both sides, and only 45
mostly on the right. Hence we see that faculsB are on an aver-
age to the left of their accompanying spots. The roost obvious
explanation of this would be that the faculas of a spot have
been uplifted from the very area occupied by that spot, and
have fallen behind to the left from being thrown up into a re-
gion of greater velocity of rotation. All this is quite in accord-
ance with our hypothesis regarding the nature of faculas. We
would likewise here remind our readers that we know from the
observations of KirchhofT that the sun's atmosphere contains va-
pors of substances, such as iron, which are condensed into the
liquid or solid state at a comparatively high temperature. Now
is it not natural to suppose tnat in the sun's photosphere we do
really see such vapors so condensed, and very unnatural to im-
agine that such vapors are seldom or never condensed, and that
what we really see is an incandescent plain underlying these
vapors?

27. Let us now attempt to answer the third question: Is a

Am. Joub. ScL-4iicoHi> Sniss, Vol. XLm, Ko. 18&— Marcb, 1967.
95



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liM Re9earehe$ on Solar Pk$fHcs»

(Spot inclttding both umbra and penumbra* a phenom^cHi whioh
takeB place beneath the level of the sun's photosphere or abore
it? To decide this question, let us state that there, are a good
many instances in which a spot breaks up in the following man-
ner. A bridge of luminous matter of the same apparent lumin-
osity as the surrounding photosphere, and unaccompanied by
any penumbra, appears to cross over the umbra or center of a
spot. There is good reason to think that this bridge is really
above the spot; for were the umbra an opaque cloud, and the
penumbra a semiopaque cloud, both being above the sun's pho*
tosphere, it is unliKely that the spot would break up in such a
manner that the terrestrial observer should not perceive some
penumbra accompanying the luminosity. Again, detached por-
tions of luminous matter appear to move across a spot without
producing any permanent alteration. We are on these accounts
disposed to think that a spot including both umbra and penum-
bra is' a phenomenon which takes place beneath the level of the
brighter part of the sun's photosphere.

28. Let us here recapitulate the answers we have given to our
three ciuestions.

(1.) The umbra of a spot is nearer the sun's center than its
penumbra, or, in other words, it is at a lower level.

(2.) Solar faculae, and probably also the whole photosphere,
consist of solid or liquid bodies of greater or less magnitude,
either slowly sinking or suspended in cequilibrio in a gaseous
medium.

(8.) A spot including both umbra and penumbra is a phe-
nomenon which takes place beneath the level of the sun's pho-
tosphere.

§ VII. Concluding remarks.

29. It would thus appear that the central part of a spot is
nearer the sun's center than the penumbra, and that both the
umbra and penumbra are probably beneath the general level of
the surrounding photosphere. Now the umbra or lowest part
of a ispot is much less luminous than the general photosphere.
But what does this probably imply, according to the laws with
which we are acquainted ? It implies that in a spot there is
probably some matter of a lower temperature than the photo-
sphere. For is it not now recognizee! as a law, that if a sub-
stance, or combination of substances, of indefinite thickness and
surface of small refieoting power have all its particles at a cer-
tain fixed temperature, this substance will give out nearly all
the rays of heat belonging to that temperature? Now the sun,
even when we look into a spot, is certainly a substance of in-
definite thickness ; and since a spot appears much less luminous
than the ordinary surSeu^e, ought we not to conclude either that
we there view matter of a lower temperaturo than the ordinary



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Retearchn on Sohtr Physics. 191

sar&ce^ or that the matter which appears within a spot has a
veiy high reflectiDg power compared to the ordinary matter of
the photosphere? This last supposition is an nnlikely one, and
the probability is that in a spot we view matter of a lower tem-
perature than the photosphere.

30. Presuming this to be the case, it appears to imply one of
ihi>ee things.

(1.) Either the general body of the sun at the level of the
bottom of a spot is of a lower temperature than the photosphere ;

(2.) Or the lower temperature is produced by some chemical
or molecular process which takes place when a spot is formed ;

(3.) Or it is produced by matter coming from a colder region.

The first of these suppositions will not be generally received
unless we are fairly driven to accept it.

The second hypothesis has already been started to account for
the lower temperature of a spot ; but we think that, according
to the laws by which we should be guided in receiving or re-
jecting an explanation in a case of this nature, this idea ought
to be rejected.

No doubt, if we knew of a case of the production of low tem-
perature, and had at the same time an independent proof of
some chemical or molecular process, such as evaporation, it
would be quite allowable for us to associate the chemical or
molecular process with the production of cold as at any rate the
most likely hypothesis ; but we do not advance in our explana-
tion of the low temperature by attributing it to an imaginary
process of the existence of which we have no^proof, and which
IS equally mysterious with the phenomenon for which it is sup-
posed to account Bather let us see if this reduction of temper-
ature can be explained by any other phenomenon of the exist-
ence of which we have independent evidence. This leads us to
consider the third hypothesis, which supposes that the reduction
is produced by matter coming from a colder regiou. Now, in
the first place, we have such a region in the atmosphere above
the photosphere, which (Art. 19) we have shown to be of a
lower temperature than the photosphere itself. Again, the ob-
servations of Chacornac and Lockyer on the behavior of the
matter surrounding a spot appear to suggest the existence of a
downward current, which is therefore a current from the colder
regions above.* On the other hand, the proper motion of spots
observed by Carrington is in favor of this nypothesis, since a
current coming from a region of greater to a region of less ab-
solute velocity of rotation would be carried on forward, and
most so nearest the equator; and this is precisely the motion of

* Does not the obserration by Lockyer of the &cu1a "ffiviog out" appear also
to indicate that the lower regions of a spot are in reality hotter than the surface,
leaTing the inferior luminoaity to be aooouated for by the dovnmsh of a oold at-
DMwpliere from aboTet



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192 Re$earche$ on Solar Pkyticn.

rts observed by Garrington. Again, we have seen (Art. 26)
t the faculsd fall behind ; so that we may imagine two cur-
rents to be engaged in the formation of a spot, — the one an as-
cending current carrying the hot matter behind, the other a de-
scending current carrying the cold matter forward. One advan-
tage of this explanation is that all the gradations of darkness,
from the faculae to the central umbra, are thus supposed to be
due to the same cause — namel v, the presence to a greater or less
extent of a comparatively cold absorbing atmosphere.

81. It is but just to ourselves and to M. Faye, to mention that
both have imagined the phenomenon of sun-spots to be due to
ascending and descending currents. M. Faye's hypothesis was
published a little before ours ; but we shall readily be believed
when we state that an idea of this kind presided over the con*
struetion of table iii, in which we have proved that the facul»
are, on an average, to the left of their accompanying spots. It
was not, however, until a short time before the publication of
the abstract of this paper by the Boyal Society, that, by discuss-
ing the subject together, we had matured our views so far as
to connect the descending current, not onlv with Garrington's
proper motion, but also with the presumed lower temperature
of a spot In this last respect our hypothesis differs entirely
from that of M. Faye, who does not imagine that the inferior
luminosity of a spot indicates the presence of matter at' a lower
temperal;ure than the photosphere.

82. In conclusion, we would venture to suggest that if the
photosphere of the sun be the plane of condensation of gaseous
matter, this plane may be found to be subject to periodical ele«
vations and depressions irji the solar atmosphere. It may be
that at the epoch of minimum spot^frequency this plane is up-
lifted very high in the solar atmosphere, so that there is com-



Online LibraryJohn AlmonThe American journal of science and arts → online text (page 22 of 102)