_ROUGH WAYS MADE SMOOTH_
ROUGH WAYS MADE SMOOTH
A SERIES OF
Familiar Essays on Scientific Subjects
BY
RICHARD A. PROCTOR
[Illustration]
_NEW IMPRESSION_
LONGMANS, GREEN, AND CO.
39 PATERNOSTER ROW, LONDON
NEW YORK AND BOMBAY
1903
_All rights reserved_
'_Let knowledge grow from more to more_'
TENNYSON
PREFACE.
It is scarcely necessary for me to explain the plan of the present
work, because I have already--in introducing my 'Light Science
for Leisure Hours,' my 'Science Byways,' and my 'Pleasant Ways in
Science'--described the method on which, as I think, such treatises as
the present should be written. This work deals with similar subjects in
a similar way; but I think the experience I have acquired in writing
other works on the same plan has enabled me to avoid some defects in
the present work which I have recognised in the others.
The list of subjects indicates sufficiently the range over which the
present volume extends. Some of them might be judged by their names to
be in no way connected with science, but it will be found that none
have been treated except in their scientific significance, though in
familiar and untechnical terms.
RICHARD A. PROCTOR.
S.S. 'ARIZONA,' IRISH SEA
_October 18, 1879._
CONTENTS.
PAGE
THE SUN'S CORONA AND HIS SPOTS 1
SUN-SPOTS AND COMMERCIAL PANICS 26
NEW PLANETS NEAR THE SUN 32
RESULTS OF THE BRITISH TRANSIT EXPEDITIONS 58
THE PAST HISTORY OF OUR MOON 81
A NEW CRATER IN THE MOON 98
THE NOVEMBER METEORS 111
EXPECTED METEOR SHOWER 117
COLD WINTERS 125
OXFORD AND CAMBRIDGE ROWING 148
ROWING STYLES 169
ARTIFICIAL SOMNAMBULISM 178
HEREDITARY TRAITS 205
BODILY ILLNESS AS A MENTAL STIMULANT 236
DUAL CONSCIOUSNESS 259
ELECTRIC LIGHTING 289
ROUGH WAYS MADE SMOOTH.
_THE SUN'S CORONA AND HIS SPOTS._
One of the most important results of observations made upon the eclipse
of July 29, 1878, indicates the existence of a law of sympathy, so to
speak, between the solar corona and the sun-spots. The inquiry into
this relation seems to me likely to lead to a very interesting series
of researches, from which may possibly result an interpretation not
only of the relation itself, should it be found really to exist, but
of the mystery of the sun-spot period. I speak of the sun-spot period
as mysterious, because even if we admit (which I think we cannot do)
that the sun-spots are produced in some way by the action of the
planets upon the sun, it would still remain altogether a mystery
how this action operated. When all the known facts respecting the
sun-spots are carefully considered, no theory yet advanced respecting
them seems at all satisfactory, while no approach even has been
made to an explanation of their periodic increase and diminution in
number. This seems to me one of the most interesting problems which
astronomers have at present to deal with; nor do I despair of seeing
it satisfactorily solved within no very long interval of time. Should
the recognition of a sympathy between the corona and the sun-spots be
satisfactorily established, an important step in advance will have been
made,--possibly even the key to the enigma will be found to have been
discovered.
I propose now to consider, first, whether the evidence we have on this
subject is sufficient, and afterwards to discuss some of the ideas
suggested by the relations which have been recognised as existing
between the sun-spots, the sierra, the coloured prominences, and the
zodiacal light.
The evidence from the recent eclipses indicates beyond all possibility
of doubt or question, that during the years when sun-spots were
numerous, in 1870 and 1871, the corona, at least on the days of the
total solar eclipses in those years, presented an appearance entirely
different from that of the corona seen on July 29, 1878, when the sun
was almost free from spots. This will be more fully indicated further
on. At present it is necessary to notice only (1) that whereas in 1870
and 1871 the inner corona extended at least 250,000 miles from the
sun, it reached only to a height of some 70,000 miles in 1878; (2) in
1870 and 1871 it possessed a very complicated structure, whereas in
1878 the definite structure could be recognised only in two parts of
the inner corona; (3) in 1871 the corona was pink, whereas in 1878
it was pearly white; (4) the corona was ten times brighter in 1871
than in 1878; lastly, in 1871 the light of the corona came in part
from glowing gas, whereas in July, 1878, the light came chiefly, if
not wholly, from glowing solid or liquid matter. I must here point
out, that the evidence of change, however satisfactory in itself,
would be quite insufficient to establish the general theory that the
corona sympathises with the solar photosphere in the special manner
suggested by the recent eclipse observations. There are few practices
more unscientific, or more likely to lead to erroneous theorising, than
that of basing a general theory on a small number of observations.
In this case we have, in fact, but a single observed correspondence,
though the observations establishing it form a series. It has been
shown that so far as the special sun-spot period from the minimum of
1867 to the minimum of 1878 is concerned, there has been a certain
correspondence between the aspect of the corona and the state of
the sun's surface, with regard to spots. To assume from that single
correspondence that the corona and the sun-spots are related in the
same way, would be hazardous in the extreme. We may indeed find, when
we consider other matters, that the probability of a general relation
of this sort existing is so great antecedently, that but slight direct
evidence would be required to establish the existence of the relation.
But it must be remembered that before the eclipse of 1878 was observed,
with the special result I have noticed, few were bold enough to assert
the probable existence of any such relationship; and certainly no one
asserted that the probability was very strong. I believe, indeed, that
no one spoke more definitely in favour of the theory that the corona
probably sympathises with the sun-spots than I did myself before the
recent eclipse; but certainly I should not then have been willing to
say that I considered the evidence very strong.
We must then look for evidence of a more satisfactory kind.
Now, although during the two centuries preceding the invention of the
spectroscope and the initiation of the solar physical researches now in
progress, observations of eclipses were not very carefully conducted,
yet we have some records of the appearance of the corona on different
occasions, which, combined with the known law of sun-spot periodicity,
may enable us to generalise more safely than we could from observations
during the present spot-period, though these observations have been far
more exact than the older ones. I propose to examine some of these.
Necessarily I must make some selection. I need hardly say that even
if there were no such relation as that which seems to be indicated by
recent observations, and if my purpose were simply to prove, either
that such a relation exists or that it does not, I could very readily
bring before the reader of these pages what would seem like the most
satisfactory evidence that the relation is real. I must ask him to
believe, however, that my purpose is to ascertain where the truth lies.
I shall neither introduce any observation of the corona because it
seems specially favourable to the theory that the corona sympathises
with the photosphere, nor omit any, because it seems definitely opposed
to that theory. To prevent any possibility of being unconsciously
prejudiced, I shall take a series of coronal observations collected
together by myself, on account of their intrinsic interest, several
years ago, when I had not in my thoughts any theory respecting periodic
changes in the corona--the series, namely, which is included in the
sixth chapter of my treatise on the sun. Each of these observations
I shall consider in connection with the known condition of the sun
as to spots, and those results which seem to bear _clearly_, whether
favourably or unfavourably, on the theory we are enquiring into, I
shall bring before the reader.
Kepler, whose attention had been specially drawn to the subject of the
light seen round the sun during total eclipse, by certain statements
which Clavius had made respecting the eclipse of 1567, describes the
eclipse of 1605 in the following terms:--'The whole body of the sun
was completely covered for a short time, but around it there shone a
brilliant light of a reddish hue and uniform breadth, which occupied a
considerable portion of the heavens.' The corona thus seen may fairly
be assumed to have resembled in extent that of 1871. A bright corona,
reaching like that seen during the eclipse of July 1878 to a height
of only about 70,000 miles from the sun's surface, would certainly
not have been described by Kepler as occupying a considerable portion
of the heavens, for a height of 70,000 miles would correspond only to
about a twelfth of the sun's diameter; and a ring so narrow would be
described very differently. It seems, then, that in 1605 a corona was
seen which corresponded with that observed when the sun has had many
spots on his surface. Now we have no record of the condition of the
sun with regard to spots in 1605; but we know that the year 1615 was
one of many spots, and the year 1610 one of few spots; whence we may
conclude safely that the year 1605 was one of many spots. This case
then is in favour of the theory we are examining.
In passing we may ask whether the observation by Clavius which had
perplexed Kepler, may not throw some light on our subject. Clavius says
that the eclipse of 1567 which should have been total was annular. The
usual explanation of this has been that the corona was intensely bright
close to the sun. And though Kepler considered that his own observation
of a broad reddish corona satisfactorily removed Clavius's difficulty,
it seems tolerably clear that the corona seen by Clavius must have
been very unlike the corona seen by Kepler. In fact the former must
have been like the corona seen in July, 1878, much smaller than the
average, but correspondingly increased in lustre. Now with regard to
the sun-spot period we can go back to the year 1567, though not quite
so securely as we could wish. Taking the average sun-spot period at
eleven years, and calculating back from the minimum of spots in the
year 1610, we get four years of minimum solar disturbance, 1599, 1588,
1577, and 1566. We should have obtained the same result if we had used
the more exact period, eleven one-ninth years, and had taken 1610·8 for
the epoch of least solar disturbance (1610·8 meaning about the middle
of October, 1610). Thus the year 1567 was a year of few sun-spots,
probably occupying almost exactly the same position in the sun spot
period as the year 1878. Clavius's observation, then, is in favour of
our theory.
But another observation between Clavius's and Kepler's may here be
noticed. Jensenius, who observed the eclipse of 1598 at Torgau in
Germany, noticed that, at the time of mid-totality, a bright light
shone round the moon. On this occasion, remarks Grant, the phenomenon
was generally supposed to arise from a defect in the totality of the
eclipse, though Kepler strenuously contended that such an explanation
was at variance with the relation between the values of the apparent
diameters of the sun and moon as computed for the time of the eclipse
by aid of the solar and lunar tables. The corona, then, must have
resembled that seen by Clavius, and since the year 1598 must have been
very near the time of fewest spots, this observation accords with the
theory we are examining.
The next observation is that made by Wyberd during the eclipse of 1652.
Here there is a difficulty arising from the strange way in which the
sun-spots behaved during the interval from 1645 to 1679. According
to M. Wolf, whose investigation of the subject has been very close
and searching, there was a maximum of sun-spots in 1639 followed
by a minimum in 1645, the usual interval of about six years having
elapsed; but there came a maximum in 1655, ten years later, followed
by a minimum in 1666, eleven years later, so that actually twenty-one
years would seem to have elapsed between successive minima (1645 and
1666). Then came a maximum in 1675, nine years later, and a minimum in
1679, four years later. Between the maxima of 1639 and 1675, including
two spot periods, an interval of thirty-six years elapsed. There is
no other instance on record, so far as I know, of so long an interval
as this for two spot-periods. In passing, I would notice how little
this circumstance accords with the theory that the sun-spots follow
an exact law, or that from observations of the sun, means can ever be
found for forming a trustworthy system of weather prediction, even if
we assumed (which has always seemed to me a very daring assumption),
that terrestrial weather is directly dependent on the progress of the
sun-spot period. But here the irregularity of the spot changes affects
us only as preventing us from determining or even from guessing what
may have been the condition of the sun's surface in the year 1652.
This year followed by seven years a period of minimum disturbance, and
preceded by three years a period of maximum disturbance; but it would
be unsafe to assume that the sun's condition in 1652 was nearer that
of maximum than that of minimum disturbance. We must pass over Wyberd's
observations of the corona in 1652, at least until some direct evidence
as to the sun's condition shall have been obtained from the papers or
writings of the observers of that year. I note only that Wyberd saw a
corona of very limited extent, having indeed a height not half so great
as that of many prominences which have been observed during recent
eclipses. If the theory we are examining should be established beyond
dispute, we should be led to infer that the year 1652 was in reality
a year of minimum solar disturbance. Perhaps by throwing in such a
minimum between 1645 and 1666, with of course a corresponding maximum,
the wild irregularity of the sun-spot changes between 1645 and 1679
would be to some degree diminished.
We are now approaching times when more satisfactory observations were
made upon the corona, and when also we have more complete records of
the aspect of the sun's surface.
In 1706 Plantade and Capies saw a bright ring of white light extending
round the eclipsed sun to a distance of about 85,000 miles, but merging
into a fainter light, which extended no less than four degrees from
the eclipsed sun, fading off insensibly until its light was lost in
the obscure background of the sky. This corresponds unmistakably
with such a corona as we should expect only to see at a time of many
sun-spots, if the theory we are examining is sound. Turning to Wolf's
list, we find that the year 1705 is marked as a year of maximum solar
disturbance, and the year 1712 as that of the next minimum. Therefore
1706 was a year of many sun-spots--in fact, 1706 may have been the year
of actual maximum disturbance, for it is within the limits of doubt
indicated by Wolf. Certainly a corona extending so far as that which
Plantade and Capies saw would imply an altogether exceptional degree of
solar disturbance, if the theory we are considering is correct.
In 1715 Halley gave the following description of the corona:--'A few
seconds before the sun was all hid, there discovered itself round the
moon a luminous ring about a digit' (a twelfth) 'or perhaps a tenth
part of the moon's diameter in breadth. It was of a pale whiteness or
rather pearl colour, seeming to me a little tinged with the colours
of the Iris, and to be concentric with the moon.' He added that the
ring appeared much whiter and brighter near the body of the moon
than at a distance from it, and that its exterior boundary was very
ill-defined, seeming to be determined only by the extreme rarity of
the luminous matter. The French astronomer Louville gave a similar
account of the appearance of the ring. He added, however, that 'there
were interruptions in its brightness, causing it to resemble the
radial glory with which painters encircle the heads of the saints.'
The smallness of the corona on this occasion corresponds with the
description of the corona seen in July 1878; and though Louville's
description of gaps is suggestive of a somewhat different aspect,
yet, on the whole, the corona seen in 1715 more closely resembles
one which would be seen at a time of minimum solar disturbance, if
our theory can be trusted, than one which would be seen at a time of
maximum disturbance. Wolf's list puts the year 1712 as one of minimum
disturbance, with one year of doubt either way, and the middle of the
year 1817 as the epoch of maximum disturbance, with a similar range of
uncertainty. The case, then, is doubtful, but on the whole inclines
to being unfavourable. I may remark that because of its unfavourable
nature, I departed from the rule I had set myself, of taking only the
cases included in my treatise on the sun. For the corona of 1715 is not
described in that treatise, as indeed affording no evidence respecting
this solar appendage. The evidence given in this case is probably
affected in some degree by the unfavourable atmospheric conditions
under which Halley certainly, and Louville probably, observed the
eclipse. In any case the evidence is not strong; only I would call
attention here to the circumstance that if, as we proceed, we _should_
come to a case in which the evidence is plainly against the theory
we are examining, we must give up the theory at once. For one case
of discordance does more to destroy a theory respecting association
between such and such phenomena, than a hundred cases of agreement
would do in the way of confirming it.
In 1724, Maraldi noticed that the corona was broadest first on the side
towards which the moon was advancing, and afterwards on the side which
the moon was leaving. From this we may infer that the corona was only a
narrow ring on that occasion, since otherwise the slight difference of
breadth due to the moon's eccentric position at the beginning and end
of totality would not have been noticeable. Now, the year 1723 was one
of minimum disturbance, with one year of doubt either way. Thus 1724
was certainly a year of few sun-spots, and may have been the actual
year of minimum disturbance. The corona then presented an appearance
according with the theory we are considering.
Few eclipses have been better observed than that of the year 1733.
The Royal Society of Sweden invited all who could spare the time to
assist, as far as their ability permitted, in recording the phenomena
presented during totality. The pastor of Stona Malm states that at
Catherinesholm, there was a ring around the sun about 70,000 miles in
height. (Of course these are not his exact words; what he actually
stated was that the ring was about a digit in breadth.) This is the
exact height assigned to the coronal ring by the observers of the
eclipse of last year. The ring seemed to be of a reddish colour.
Another clergyman, Vallerius, states also that the ring was of this
colour, but adds that at a considerable distance from the sun it had
a greenish hue. This suggests the idea that the outer corona was seen
also by Vallerius, and that it had considerable breadth. The reddish
colour of the inner light portion would correspond to the colour it
would have if it consisted in the main of glowing hydrogen. If that
really was its constitution, then the theory advanced by one observer
of the last eclipse, that at the time of minimum solar disturbance the
glowing hydrogen is withdrawn from the corona, would be shown to be
incorrect. For 1733 was the actual year of minimum solar disturbance.
The pastor of Smoland states that 'during the total obscuration the
edge of the moon's disc resembled gilded brass, and the faint ring
round it emitted rays in an upward as well as in a downward direction,
similar to those seen beneath the sun when a shower of rain is
impending.' The mathematical lecturer of the Academy of Charles-stadt,
M. Edstrom, observed these rays with special attention: he says that
'they plainly maintained the same position, until they vanished along
with the ring upon the re-appearance of the sun.' On the other hand,
at Lincopia no rays were seen. On the whole it seems clear from the
accounts of this eclipse that the inner corona was bright and narrow;
rays issued from the outer faint ring; but they were very delicate
phenomena, easily concealed by atmospheric haze, and thus were not
everywhere observed. As rays were seen in July 1878, there is nothing
in the evidence afforded by the eclipse of 1733, occurring at a time
of few spots, which opposes itself definitely to the theory we are
considering. But the reddish colour of the corona as already noticed
is a doubtful feature: in July, 1878, the bright inner corona was of a
pearl colour and lustre.
During the eclipse of February, 1766, the corona presented four
luminous expansions, and seems to have presented a greater expansion
than we should expect in a year of minimum solar disturbance. Such,
however, the year 1766 certainly was. The evidence in this case is
unfavourable to our theory--not quite decisively so, but strongly.
For we should expect that in the year of actual minimum disturbance
the corona would be even narrower than in the year 1878, which was
the year following that of least disturbance. And again, a strongly
distinctive feature in the corona of July, 1878, was the absence of
wide expansions, such as were seen in 1870 and 1871. Now if this
peculiarity should really be attributed to the relation existing
between the corona and the sun-spots, we should infer that in 1766
the corona would have been still more markedly uniform in shape. The
existence of four well marked expansions on that occasion forces us
to assume that either the relation referred to has no real existence,
or else that the corona may change from week to week as the condition
of the sun's surface changes, and that in February, 1766, the sun was
temporarily disturbed, though the year, as a whole, was one of minimum
disturbance. But as the epoch of actual minimum was the middle of 1766,
February 1766 should have been a time of very slight disturbance. I
do not know of any observations of the sun recorded for the month of
February, 1766. On the whole, the eclipse of 1766 must be regarded as
throwing grave doubt on the relation assumed by our theory as existing
between the corona and the sun-spots; and as tending to suggest
that some wider law must be in question than the one we have been
considering--if any association really exists.
The account given by Don Antonio d'Ulloa of the appearance presented
by the corona during the total eclipse of 1778, is rendered doubtful
by his reference to an apparent rotatory motion of the normal rays.
He says that about five or six seconds after totality had begun, a
brilliant luminous ring was seen around the dark body of the moon.
The ring became brighter as the middle of totality approached. 'About
the middle of the eclipse, the breadth of the ring was equal to
about a sixth of the moon's diameter. There seemed to issue from it
a great number of rays of unequal length, which could be discerned
to a distance equal to the moon's diameter.' Then comes the part of
d'Ulloa's description which seems difficult to accept. He says that the
corona 'seemed to be endued with a rapid rotatory motion, which caused
it to resemble a firework turning round its centre.' The colour of
the light, he proceeds, 'was not uniform throughout the whole breadth
of the ring. Towards the margin of the moon's disc it appeared of a
reddish hue; then it changed to a pale yellow, and from the middle
to the outer border the yellow gradually became fainter, until at
length it seemed almost quite white.' Setting aside the rays and their
rotation, d'Ulloa's account of the inner corona may be accepted as
satisfactory. The height of this ring was, it seems, about 140,000
miles, or twice that of the ring seen in July 1878. As the year 1779
was one of maximum solar disturbance, there were doubtless many spots
in 1778; and the aspect of the corona accorded well with the theory
that the corona expands as the number of sun-spots increases.
We come now to three eclipses which are especially interesting as
having been all carefully observed, some observers having seen all
three,--the eclipses, namely, of 1842, 1851, and 1860. Unfortunately
the eclipses of 1842 and 1851 occurred when the sun-spots were neither
at their greatest nor at their least degree of frequency. For a maximum
of sun-spots occurred in 1837, and a minimum in 1844, so that 1842 was
on what may be called the descending slope of a sun-spot wave, nearer
the hollow than the crest, but not very near either: again, a maximum
occurred in 1848, and a minimum in 1856, so that 1851 was also on the
descending slope of a sun-spot wave, rather nearer the crest than the
hollow, but one may fairly say about midway between them. Still it is
essential in an inquiry of this sort to consider intermediate cases.
We must not only apply the _comparentia ad intellectum instantiarum
convenientium_, but also the _comparentia instantiarum secundum magis
ac minus_. If the existence of great solar disturbances causes the
corona to be greatly enlarged, as compared with the corona seen when
the sun shows no spots, we should expect to find the corona moderately
enlarged only when the sun shows a considerable but not the maximum
number of spots. And again, it is conceivable that we may find some
noteworthy difference between the aspect of the corona when sun-spots
are diminishing in number, and its aspect when they are increasing.
This point seems the more to need investigation when we note that
the evidence derived from eclipses occurring near the time either
of maximum or of minimum solar disturbance has not been altogether
satisfactory. It may be that we may find an explanation of the
discrepancies we have recognised, in some distinction between the state
of the corona when spots are increasing and when they are diminishing
in number.
It is noteworthy that several careful observers of the corona in 1842
believed that they could recognise motion in the coronal rays. Francis
Baily compared the appearance of the corona to the flickering light of
a gas illumination. O. Struve also was much struck by the appearance of
violent agitation in the light of the ring. It seems probable that the
appearance was due to movements in that part of our atmosphere through
which the corona was observed. The extent of the corona was variously
estimated by different observers. Petit, at Montpelier, assigned to
it a breadth corresponding to a height of about 200,000 miles; Baily
a height of about 500,000 miles; and O. Struve a height of more than
800,000 miles. The last-named observer also recognised luminous
expansions extending fully four degrees (corresponding to nearly seven
million miles) from the sun. Picozzi, at Milan, noticed two jets of
light, which were seen also by observers in France. Rays also were seen
by Mauvais at Perpignan, and by Baily at Paria. But Airy, observing the
corona from the Superga, could see no radiation; he says 'although a
slight radiation might have been perceptible, it was not sufficiently
intense to affect in a sensible degree the annular structure by which
the luminous appearance was plainly distinguished.' These varieties
in the aspect of the corona were doubtless due to varieties in the
condition of the atmosphere through which the corona was seen. Now it
cannot be questioned that, so far as extension is concerned, the corona
seen in 1842 was one which, if the theory we are considering were
sound, we should expect to see near the time of maximum rather than of
minimum solar disturbance. On the other hand, in brightness the corona
of 1842 resembled, if it did not surpass, that of July 1878.
'I had imagined,' says Baily, 'that the corona, as to its brilliant or
luminous appearance, would not be greater than that faint crepuscular
light which sometimes takes place (_sic_) in a summer evening, and
that it would encircle the moon like a ring. I was therefore somewhat
surprised and astonished at the splendid scene which now so suddenly
burst upon my view.'
The light of the corona was so bright, O. Struve states, that the naked
eye could scarcely endure it; 'many could not believe, indeed, that the
eclipse was total, so strongly did the corona's light resemble direct
sunlight.' Thus while as to extent the corona in 1842 presented the
appearance to be expected at the time of maximum solar disturbance, if
our theory is sound, its brightness was that corresponding to a time
of minimum disturbance. Its structure corresponded with the former
condition. The light of the corona was not uniform, nor merely marked
by radiations, but in several places interlacing lines of light could
be seen. Arago, at Perpignan, observed with the unaided eye a region
of the corona where the structure was as of intertwined jets giving an
appearance resembling a hank of thread in disorder.
Certainly, for an eclipse occurring two years from the time of minimum,
and five years from the time of maximum disturbance, that of July,
1842,[1] has not supplied evidence favouring the theory with which we
started. Whether any other theory of association between the corona and
the sun-spots will better accord with the evidence hitherto collected
remains to be seen.
Turn we now to the eclipse of 1851, occurring nearly midway between
the epochs of maximum solar disturbance (1848) and minimum solar
disturbance (1856). I take the account given by Airy, our Government
astronomer, as he was one of the observers of the eclipse of 1842.
'The corona was far broader,' he says, 'than that which I saw in
1842. Roughly speaking, the breadth was little less than the moon's
diameter, but its outline was very irregular. I did not notice any
beams projecting from it which deserved notice as much more conspicuous
than the others; but the whole was beamy, radiated in structure, and
terminated--though very indefinitely--in a way which reminded me of the
ornament frequently placed round a mariner's compass. Its colour was
white, or resembling that of Venus. I saw no flickering or unsteadiness
of light. It was not separated from the moon by any interval, nor had
it any annular structure. It looked like a radiated luminous cloud
behind the moon.'
The corona thus described belongs to that which our theory associates
with the period of maximum rather than of minimum solar disturbance.
Definite peculiarities of structure seem to have been more numerous and
better marked than in 1842. It accords with our theory that 1851 was
a year of greater solar disturbance than was observed in 1842, as the
following numbers show:--
Days of Days without New groups
observation spots observed
1842 307 64 68
1851 308 0 141
1860 332 0 211
I have included the year 1860, as we now proceed to consider the corona
then seen by Airy. The year 1860 did not differ very markedly, it will
be observed, from 1851, as regards the number of new groups of spots
observed by Schwabe, especially when account is taken of the number of
days in which the sun was observed in these two years. But 1860 was a
year of maximum solar disturbance, whereas 1851 was not.[2]
Airy remarks of the corona in 1860:--'It gave a considerable body,
but I did not remark either by eye-view or by telescope-view
anything annular in its structure; it appeared to me to resemble,
with some irregularities (as I stated in 1851), the ornament round a
compass-card.'
Bruhns of Leipsic noted that the corona shone with an intense white
light, so lustrous as to dim the protuberances. He noticed that a ray
shot out to a distance of about one degree indicating a distance of at
least 1,500,000 miles from the sun's surface. This was unquestionably
a coronal appendage as neither the direction nor the length of the ray
varied for ten seconds, during which Bruhns directed his attention to
it. Its light was considerably feebler than that of the corona, which
was of a glowing white, and seemed to coruscate or twinkle. Bruhns
assigned to the inner corona a height varying from about 40,000 to
about 80,000 miles. But this was unquestionably far short of the true
height. In fact, Secchi's photographs show the corona extending to
a distance of at least 175,000 miles from the surface of the sun.
Therefore probably what Bruhns calls the base of the corona was in
reality only the prominence region, and the inner corona was that which
he describes as varying in breadth or height from nearly one-half to
a quarter of a degree--that is from about 800,000 to about 400,000
miles. De la Rue gives a somewhat similar general description of the
corona seen in 1860. He remarks that it was extremely bright near the
moon's body, and of a silvery whiteness. The picture of the corona by
Feilitsch (given at p. 343 of my book on the Sun) accords with these
descriptions.
On the whole, the eclipse of 1860 affords evidence according well with
the theory we have been considering, except as regards the brightness
and the colour of the corona, which correspond more closely with what
was observed in July, 1878, with the lustre and colour of the corona
in 1870 and 1871. In this respect, it is singular that the eclipse of
1867, which occurred (see preceding note) when the sun spots were fewer
in number, presented a decided contrast to that of 1860,--the contrast
being, however, precisely the reverse of that which our theory would
require, if the colour and brightness of the corona be considered
essential features of any law of association.
Herr Grosch, describing the corona of 1867, says, 'There appeared
around the moon a reddish glimmering light similar to that of the
aurora, and almost simultaneously with this (I mean very shortly after
it) the corona.' It is clear, however, from what follows, that the
reddish light was what is now commonly called the inner corona, which
last July, when the sun was in almost exactly the same condition as
regards the spots, was pearly white and intensely bright. 'This reddish
glimmer,' he proceeds, 'which surrounded the moon with a border of the
breadth of at most five minutes' (about 140,000 miles) 'was not sharply
bounded in any part, but was extremely diffused and less distinct in
the neighbourhood of the poles.' Of the outer corona he remarks that
'its apparent height amounted to about 280,000 miles opposite the
solar poles, but opposite the polar equator to about 670,000 miles. Its
light was white. This white light was not in the least radiated itself,
but it had the appearance of rays penetrating through it; or rather as
if rays ran over it, forming symmetrical pencils diverging outwards,
and passing far beyond the boundary of the white light. These rays had
a more bluish appearance, and might best be compared to those produced
by a great electro-magnetic light. Their similarity to these, indeed,
was so striking, that under other circumstances I should have taken
them for such, shining at a great distance. The view of the corona I
have described is that seen with the naked eye.... In the white light
of the corona, close upon the moon's edge, there appeared several dark
curves. They were symmetrically arched towards the east and west,
sharply drawn, and resembling in tint lines drawn with a lead pencil
upon white paper.... Beginning at a distance of one minute (about
26,000 miles), they could be traced up to a distance of about nine
minutes (some 236,000 miles) from the moon's edge.'
Almost all the features observed in this case correspond closely with
those noted and photographed during the eclipse of December, 1871.
In other words the corona seen in 1867, when the sun was passing
through the period of least solar disturbance, closely resembled the
corona seen in 1871, when the sun was nearly in its stage of greatest
disturbance. Even the spectroscopic evidence obtained in 1871 and July,
1878, may be so extended as to show with extreme probability what would
have been seen in 1867 if spectroscopic analysis had then been applied.
We cannot doubt that the reddish inner corona, extending to a height of
about 140,000 miles, would have been found under spectroscopic analysis
to shine in part with the light of glowing hydrogen, as the reddish
corona of 1871 did. The white corona of July, 1878, on the contrary,
shone only with such light as comes from glowing solid or liquid
matter. Here then, again, the evidence is unfavourable to our theory;
for the corona in 1867 should have closely resembled the corona of
1878, if this theory were sound.
It would be idle, I think, to seek for farther evidence either in
favour of the theory we originally proposed to discuss, or against it:
for the evidence of the eclipse of 1867 disposes finally of the theory
in that form. I may note in passing that the eclipse of 1868 gave
evidence almost equally unfavourable to the theory, while the evidence
given by the eclipse of 1869 was neutral. It will be desirable,
however, to consider, before concluding our inquiry, the evidence
obtained in 1871 and last July, in order that we may see what, after
all, that evidence may be regarded as fairly proving with regard to
coronal variations.
First, however, as I have considered two eclipses which occurred when
the sun spots were decreasing in number--namely, those of 1842 and
1851, midway (roughly speaking) between the crest and hollow of the
sun-spot wave on its descending slope, it may be well to consider an
eclipse which was similarly situated with respect to the ascending
slope of a sun-spot wave. I take, then, the eclipse of 1858, as seen in
Brazil by Liais. The picture drawn by this observer is one of the most
remarkable views of the corona ever obtained. It is given at p. 339 of
my book on the Sun. Formerly it was the custom to deride this drawing,
but since the eclipse of 1871, when the corona was photographed, it
has been admitted that Liais's drawing may be accepted as thoroughly
trustworthy. It shows a wonderfully complex corona, like that of 1871,
extending some 700,000 miles from the sun, and corresponding in all
respects with such a corona as our theory (if established) would have
associated with the stage of maximum solar disturbance. As in this
respect the eclipse of 1858, when sun-spots were increasing, resembled
those of 1842 and 1851, when sun-spots were diminishing in number,
we find no trace of any law of association depending on the rate of
increase or diminution of solar disturbance.
If we limited our attention to the eclipses of 1871 and of July, 1878,
we should unquestionably be led to adopt the belief that the corona
during a year of many spots differs markedly from the corona when the
sun shows few spots, or none. So far as the aspect of the corona is
concerned, I take the description given by the same observer in both
cases, as the comparison is thus freed as far as possible from the
effect of personal differences.
Mr. Lockyer recognised in 1871 a corona resembling a star-like
decoration, with its rays arranged almost symmetrically--three
above and three below two dark spaces or rifts at the extremity of
a horizontal diameter. The rays were built up of innumerable bright
lines of different length, with more or less dark spaces between
them. Near the sun this structure was lost in the brightness of the
central ring, or inner corona. In the telescope he saw thousands of
interlacing filaments, varying in intensity. The rays so definite
to the eye were not seen in the telescope. The complex structure of
interlacing filaments could be traced only to a height of some five or
six minutes (from 135,000 to 165,000 miles) from the sun, there dying
out suddenly. The spectroscope showed that the inner corona, to this
height at least (but Respighi's spectroscopic observations prove the
same for a much greater distance from the sun), was formed in part
of glowing gas--hydrogen--and the vapour of some as yet undetermined
substance, shining with light of a green tint, corresponding to
1474 of Kirchhoff's scale. But also a part of the coronal light
came from matter which reflected sunlight; for its spectrum was the
rainbow-tinted streak crossed by dark lines, which we obtain from any
object illuminated by the sun's rays. It should be added that the
photographs of the corona in 1871 show the three great rays above and
three below, forming the appearance as of a star-like decoration,
described by Mr. Lockyer; insomuch as it is rather strange to find Mr.
Lockyer remarking that 'the difference between the photographic and
the visible corona came out strongly, ... and the non solar origin of
the radial structure was conclusively established.' The resemblance
is, indeed, not indicated in the rough copy of the photographs which
illustrates Mr. Lockyer's paper; but it is clearly seen in the
photographs themselves, and in the fine engraving which has been formed
from them for the illustration of the volume which the Astronomical
Society proposes to issue (some time in the present century, perhaps).
Now, in July, 1878, the corona presented an entirely different
appearance. Mr. Lockyer, in a telegram sent to the _Daily News_,
describes it as small, of pearly lustre, and having indications of
definite structure in two places only. Several long rays were seen; but
the inner corona was estimated as extending to a height of about 70,000
miles from the sun's surface. The most remarkable change, however,
was that which had taken place in the character of the corona's
spectrum--or, in other words, in the physical structure of the corona.
The bright lines or bright images of the inner corona (according as
it was examined through a slit or without one) were not seen in July,
1878, showing that no part, or at least no appreciable part, of its
light came from glowing gaseous matter. But also the dark lines seen by
Janssen in 1871 were wanting on this occasion, showing that the corona
did not shine appreciably by reflecting sunlight. The spectrum was, in
fine, a continuous rainbow-tinted streak, such as that given by glowing
solid or liquid matter.
The inference clearly is: 1. That in July, 1878, the gaseous matter
which had been present in the corona in 1871 was either entirely absent
or greatly reduced in quantity; 2. The particles of solid or liquid
(but probably solid) matter which, by reflecting sunlight, produced a
considerable portion of the corona's light in 1871, were glowing with
heat in July, 1878, and shone in the main with this inherent light;
and 3. The entire corona was greatly reduced in size in July, 1878, as
compared with that which formed the 'star-like decoration' around the
black body of the moon in December, 1871.
We cannot, however, accept the theory that such a corona as was seen
in 1871 invariably surrounds the sun in years of great disturbance,
while the corona of last month is the typical corona for years of
small solar disturbance. The generalisation is flatly contradicted
by the evidence which I have presented in the preceding pages. It
may be that such a corona as was seen in 1871 is common in years of
great disturbance, just as spots are then more common, though not
always present; while such a corona as was seen in July, 1878, is more
common in years of small disturbance, just as days when the sun is
wholly without spots are then more common, though from time to time
several spots, and sometimes very large spots, are seen in such years.
On the whole, I think the evidence I have collected favours rather
strongly the inference that an association of this sort really exists
between the corona and the sun-spots. It would, however, be unsafe at
present to generalise even to this extent; while certainly the wide
generalisation telegraphed to Europe from America as the great result
of the eclipse observations in July, 1878, must unhesitatingly be
rejected.
It remains to be considered how science may hope to obtain more
trustworthy evidence than we yet have respecting the corona and its
changes of form, extent, lustre, and physical constitution. In the case
of the prominences, we have the means of making systematic observations
on every fine, clear day. It has been, indeed, through observations
thus effected by the spectroscopic method that an association has been
recognised between the number, size, and brilliancy of the prominences
on the one hand, and the number, size, and activity of the sun-spots
on the other. But in the case of the corona, we are as yet unable to
make any observations except at the time of total solar eclipse. It
seems almost impossible to hope that any means can be devised for
seeing the corona at any other time. Of course, without the aid of the
spectroscope the corona, as ordinarily seen during total eclipses, must
be entirely invisible when the sun is shining in full splendour. No one
acquainted with even the merest elements of optics could hope to see
the corona with an ordinary telescope at such a time. The spectroscope,
again, would not help in the slightest degree to show such a corona as
was shining in July, 1878. For the power of the spectroscope to show
objects which under ordinary conditions are invisible, depends on the
separation of rays of certain tints from the rays of all the colours
of the rainbow, which make up solar light; and as the corona in July,
1878, shone with all the colours of the rainbow, and not with certain
special tints, the power of the spectroscope would be thrown away on a
corona of that kind. All that we can ever hope to do is to discern the
gaseous corona when, as in 1871, it is well developed, by spectroscopic
appliances more effective for that purpose than any which have hitherto
been adopted; for all which have as yet been adopted have failed.
Now, the difficulty of the problem will be recognised when we remember
that the strongest tints of the corona's light--the green tint
classified as 1474 Kirchhoff--has been specially but ineffectually
searched for in the sun's neighbourhood with the most powerful
spectroscopic appliances yet employed in the study of the coloured
prominences. In other words, when the light of our own air over the
region occupied by the corona has been diluted as far as possible
by spectroscopic contrivances, the strongest of the special coronal
tints has yet failed to show through the diluted spectrum of the sky.
Again, we have even stronger evidence of the difficulty of the task in
the spectroscopic observations made by Respighi during the eclipse of
1871. The instrument, or I should rather, perhaps, say the arrangement,
which during mid totality showed the green image of the corona to a
height of about 280,000 miles, did not show any green ring at all at
the beginning of totality. In other words, so faint is the light of the
gaseous corona, even at its brightest part, close to the sun, that the
faint residual atmospheric light which illuminates the sky over the
eclipsed sun at the beginning of totality sufficed to obliterate this
part of the coronal light.