difficulties of this observation, the gaseous corona can be rendered
discernible, remains to be seen. I must confess my own hopes that the
problem will ever be successfully dealt with are very slight, though
not absolutely evanescent. It seems to me barely possible that the
problem might be successfully attacked in the following way. Using a
telescope of small size, for the larger the telescope the fainter is
the image (because of greater loss of light by absorption), let the
image of the sun be received in a small, perfectly darkened camera
attached to the eye-end of the telescope. Now if the image of the sun
were received on a smooth white surface we know that the prominences
and the corona would not be visible. And again, if the part of such
a surface on which the image of the sun itself fell were exactly
removed, we know (the experiment has been tried by Airy) that the
prominences would not be seen on the ring of white surface left after
such excision. Still less, then, would the much fainter image of the
corona be seen. But if this ring of white surface, illuminated in
reality by the sky, by the ring of prominences and sierra, and by
the corona, were examined through a battery of prisms (used without
a slit) adjusted to any one of the known prominence tints, the ring
of prominences and sierra would be seen in that special tint. If the
battery of prisms were sufficiently effective, and the tint were one of
the hydrogen tints--preferably, perhaps, the red--we might possibly be
able to trace the faint image of the corona in that tint. But we should
have a better chance with the green tint corresponding to the spectral
line 1474 Kirchhoff. If the ring of white surface were replaced by a
ring of green surface, the tint being as nearly that of 1474 Kirchhoff
as possible, the chance of seeing the coronal ring in that tint would
be somewhat increased; and, still further, perhaps, if the field of
view were examined through green glass of the same tint. It seems just
possible that if prisms of triple height were used, through which the
rays were carried three times, by an obvious modification of the usual
arrangement for altering the level of the rays, thus giving a power of
eighteen flint glass prisms of sixty degrees each, evidence, though
slight perhaps, might be obtained of the presence of the substance
which produces the green line. Thus variations in the condition of
the corona might be recognised, and any law affecting such variations
might be detected. I must confess, however, that a consideration of
the optical relations involved in the problem leads me to regard the
attempt to recognise any traces of the corona when the sun is not
eclipsed as almost hopeless.
It is clear that until some method for thus observing the corona has
been devised, future eclipse observations will acquire a new interest
from the light which they may throw on the coronal variations, and
their possible association in some way, not as yet detected, with the
sun-spot period. Even when a method has been devised for observing the
gaseous corona, the corona whose light comes either directly or by
reflection from solid or liquid matter will still remain undiscernible
save only during total eclipses of the sun. Many years must doubtless
pass, then, before the relation of the corona to the prominences and
the sun-spots shall be fully recognised. But there can be no question
that the solution of this problem will be well worth waiting for, even
though it should not lead up (as it most probably will) to the solution
of the mystery of the periodic changes which affect the surface of the
sun.
FOOTNOTES:
[Footnote 1: The actual condition of the sun in 1842 may be inferred
from the following table, showing the number of spots observed in 1837
the preceding year of maximum disturbance, in 1842, and in 1844 the
following year of minimum disturbance; the observer was Schwabe of
Dessau:
Days of Days without New groups
observation spots observed
1837 168 0 333
1842 307 64 68
1844 321 111 52
Only it should be noticed that nearly all the spots seen in the year
1844 belonged to the next period, the time of actual minimum occurring
early in 1844.]
[Footnote 2: The following table shows the position occupied by the
years 1851 and 1860 in this report, as compared with the year 1848
(maximum next preceding 1851), 1856 (minimum next following 1851) and
1867, minimum next following 1860:--
Days of Days without New groups
observation spots observed
1848 278 0 930
1851 308 0 141
1856 321 193 34
1860 332 0 211
1867 312 195 25
A comparison of the three tables given in these notes and the text will
afford some idea of the irregularities existing in the various waves of
sun-spots.]
_SUN-SPOTS AND COMMERCIAL PANICS._
We are not only, it would seem, to regard the sun as the ultimate
source of all forms of terrestrial energy, existent or potential, but
as regulating in a much more special manner the progress of mundane
events. Many years have passed since Sabine, Wolf, and Gauthier
asserted that variations in the daily oscillations of the magnetic
needle appear to synchronise with the changes taking place in the
sun's condition, the oscillations attaining their _maximum_ average
range in years when the sun shows most spots, and their _minimum_
range when there are fewest spots. And although it is well known that
the Astronomer Royal in England and the President of the Academy
of Sciences in France reject this doctrine, it still remains in
vogue. True, the average magnetic period appears to be about 10.45
years, while Wolf obtains for the sun-spot period 11.11 years; but
believers in the connection between terrestrial magnetic disturbances
and sun-spots consider that among the imperfect records of the past
condition of the sun Wolf must have lost sight of one particular wave
of sun-spots, so to speak. If there have been 24 such waves between
1611 and 1877, when sun-spots were fewest, we get Wolf's period
of 11.11 years; if there have been 25 such waves then, taking an
admissible estimate for the earliest epoch, we get 10.45 years, the
period required to synchronise with the period of terrestrial magnetic
changes. The matter must be regarded as still _sub judice_. This,
however, is only one relation out of many now suggested. Displays of
the aurora, being unquestionably dependent on the magnetic condition
of the earth, would of course be associated with the sun spot period,
if the magnetic period is so; and certainly the most remarkable
displays of the aurora in recent times have occurred when the sun has
shown many spots. Yet this of itself proves nothing more than had
been already known--namely, that the last few magnetic periods have
nearly synchronised with the last few sun-spot periods. It is rather
strange, too, that no auroras are mentioned in the English records
for 80 years preceding the aurora of 1716, and in the records of the
Paris Academy of Sciences one only--that of 1666, which occurred when
sun-spots were fewest. The great aurora of 1723, seen as far south
as Bologna, also occurred at the time of _minimum_ solar activity.
Here we are not depending on either Wolf's period of 11 years or
Brown's of 10-1/2 years; from records of actual observation it is
known that in 1666 and 1713 there were no sun-spots. In fact it is
worth mentioning that Cassini, writing in 1671, says, 'It is now about
20 years since astronomers have seen any considerable spots on the
sun,' a circumstance which throws grave doubt on the law of sun-spot
periodicity itself. It is at least certain that the interval from
_maximum_, spot-frequency to _maximum_, or from _minimum_ to _minimum_,
has sometimes fallen far short of 9 years, and has at others exceeded
18 years.
It appears again that certain meteorological phenomena show a tendency,
more or less marked, to run through a ten-year cycle. Thus, from the
records of rainfall kept at Oxford it appears that more rain fell under
west and south-west winds when sun-spots were largest and most numerous
than under south and south-east winds, these last being the more rainy
winds when sun-spots were least in size and fewest in number. This
is a somewhat recondite relation, and at least proves that earnest
search has been made for such cyclic relations as we are considering.
But this is not all. When other records were examined, the striking
circumstance was discovered that elsewhere, as at St. Petersburg, the
state of things observed at Oxford was precisely reversed. At some
intermediate point between Oxford and St. Petersburg, no doubt the
rainfall under the winds named was equally distributed throughout the
spot period. Moreover, as the conditions thus differ at different
places, we may assume that they differ also at different times. Such
relations appear then to be not only recondite, but complicated.
When we learn that during nearly two entire sun-spot periods cyclones
have been somewhat more numerous in the Indian Seas when spots are
most numerous than when the sun is without spots, and _vice versâ_,
we recognise the possible existence of cyclic relations better worth
knowing than those heretofore mentioned. The evidence is not absolutely
decisive; some, indeed, regard it as scarcely trustworthy. Yet there
does seem to have been an excess of cyclonic disturbance during the
last two periods of great solar disturbance, precisely as there was
also an excess of magnetic disturbance during those periods. The
excess was scarcely sufficient, however, to justify the rather daring
statement made by one observer, that 'the whole question of cyclones
is merely a question of solar activity.' We had records of some very
remarkable cyclonic disturbances during the years 1876 and 1877, when
the sun showed very few spots, the actual _minimum_ of disturbance
having probably been reached late in 1877. A prediction that 1877 would
be a year of few and slight storms would have proved disastrous if
implicit reliance had been placed on it by seamen and travellers.
Rainfall and atmospheric pressure in India have been found to vary
in a cyclic manner, of late years at any rate, the periods being
generally about 10 or 11 years. The activity of the sun, as shown by
the existence of many spots, apparently makes more rainfall at Madras,
Najpore, and some other places; while at Calcutta, Bombay, Mysore,
and elsewhere it produces a contrary effect. Yet these effects are
produced in a somewhat capricious way: for sometimes the year of actual
_maximum_ spot frequency is one in which rainfall is below the average
(instead of above) at the former stations, and above the average
(instead of below) at the latter. It is only by taking averages--and in
a somewhat artificial manner--that the relation seems to be indicated
on which stress has been laid.
Since Indian famines are directly dependent on defective rainfall, it
is natural that during the years over which observation has hitherto
extended the connection apparently existing between sun-spots and
Indian rainfall should seem also to extend itself to Indian famines. It
was equally to be expected that since cyclones have been rather more
numerous, for some time past, in years when sun-spots have been most
numerous, shipwrecks should also have been somewhat more frequent in
such years. Two years ago Mr. Jeula gave some evidence which, in his
opinion, indicated such a connection between sun-spots and shipwrecks.
He showed that in the four years of fewest spots the mean percentage
of losses was 8.64; in four intermediate years the mean percentage was
9.21; in three remaining years of the eleven-year cycle--that is, in
three years of greatest spot frequency the mean percentage was 9.53.
Some suggested that possibly such events as the American war, which
included two of the three years of greatest spot frequency, may have
had more effect than sun-spots in increasing the percentage of ships
lost; while perhaps, the depression following the commercial panic of
1866 (at a time of fewest sun-spots) may have been almost as effective
in reducing the percentage of losses as the diminished area of solar
maculation. But others consider that we ought rather to regard the
American war as yet another product of the sun's increased activity in
1860-61, and the great commercial panic of 1866 as directly resulting
from diminished sun-spots at that time, thus obtaining fresh evidence
of the sun's specific influence on terrestrial phenomena instead of
explaining away the evidence derived from Lloyd's list of losses.
This leads us to the last, and, in some respects, the most singular
suggestion respecting solar influence on mundane events--the idea,
namely, that commercial crises synchronise with the sun-spot period,
occurring near the time when spots are least in size and fewest in
number; or, as Professor Jevons (to whom the definite enunciation of
this theory is due) poetically presents the matter, that from 'the sun,
which is truly "of this great world both eye and soul," we derive our
strength and our weakness, our success and our failure, our elation in
commercial mania, and our despondency and ruin in commercial collapse.'
We have better opportunities of dealing with this theory than with the
others, for we have records of commercial matters extending as far back
as the beginning of the eighteenth century. In fact, we have better
evidence than Professor Jevons seems to have supposed, for whereas in
his discussion of the matter he considers only the probable average
of the sun-spot period, we know approximately the epochs themselves
at which the _maxima_ and _minima_ of sun spots have occurred since
the year 1700. The evidence as presented by Professor Jevons is very
striking, though when examined in detail it is rather disappointing.
He presents the whole series of decennial crises as follows:--1701?
(such query marks are his own), 1711, 1721, 1731-32, 1742 (?), 1752
(?), 1763, 1772-73, 1783, 1793, 1804-5 (?), 1815, 1825, 1836-9 (1837
in the United States), 1847, 1857, 1866 and 1878. The average interval
comes out 10.466 years, showing, as Jevons points out, 'almost perfect
coincidence with Brown's estimate of the average sun-spot period.' Let
us see, however, whether these dates correspond so closely with the
years of _minimum_ spot-frequency as to remove all doubt. Taking 5-1/4
years as the average interval between _maximum_ and _minimum_ sun-spot
frequency, we should like to find every crisis occurring within a year
or so on either side of the _minimum_ though we should prefer perhaps
to find the crisis always following the time of fewest sun-spots,
as this would more directly show the depressing effect of a spotless
sun. No crisis ought to occur within a year or so of _maximum_ solar
disturbance; for that, it should seem, would be fatal to the suggested
theory. Taking the commercial crises in order, and comparing them with
the known (or approximately known) epochs of _maximum_ and _minimum_
spot frequency, we obtain the following results (we italicize numbers
or results unfavourable to the theory):--The doubtful crisis of 1701
followed a spot _minimum_ by _three_ years; the crisis of 1711 preceded
a _minimum_ by one year; that of 1721 preceded a _minimum_ by two
years; 1731-32, preceded a _minimum_ by one year; 1742 preceded a
_minimum_ by _three_ years; 1752 followed a _maximum_ by _two_ years;
1763 followed a _maximum_ by _a year and a half_; 1772-73 came _midway_
between a _maximum_ and a _minimum_; 1783 preceded a _minimum_ by
nearly two years; 1793 came nearly midway between a _maximum_ and a
_minimum_; 1804-5 coincided with a _maximum_; 1815 preceded a _maximum_
by two years; 1825 followed a _minimum_ by _two_ years; 1836-39
_included_ the year 1837 of _maximum_ solar activity (that year being
the time also when a commercial crisis occurred in the United States);
1847 preceded a _maximum_ by a _year and a half_; 1866 preceded a
_minimum_ by a year; and 1878 followed a _minimum_ by a year. Four
favourable cases out of 17 can hardly be considered convincing. If we
include cases lying within two years of a _minimum_, the favourable
cases mount up to seven, leaving ten unfavourable ones. It must be
remembered, too, that a single decidedly unfavourable case (as 1804,
1815, 1837) does more to disprove such a theory than 20 favourable
cases would do towards establishing it. The American panic of 1873,
by the way, which occurred when spots were very numerous, decidedly
impairs the evidence derived from the crises of 1866 and 1878.
_NEW PLANETS NEAR THE SUN._
Perhaps no scientific achievement during the present century has been
deemed more marvellous than the discovery of the outermost member (so
far as is known) of the sun's family of planets. In many respects,
apart from the great difficulty of the mathematical problem involved,
the discovery appealed strongly to the imagination. A planet seventeen
hundred millions of miles from the sun had been discovered in March,
1781, by a mere accident, though the accident was not one likely to
occur to any one but an astronomer constantly studying the star-depths.
Engaged in such observation, but with no idea of enlarging the known
domain of the sun, Sir W. Herschel perceived the distant planet Uranus.
His experienced eye at once recognised the fact that the stranger
was not a fixed star. He judged it to be a comet. It was not until
several weeks had elapsed that the newly discovered body was proved
to be a planet, travelling nearly twice as far away from the sun as
Saturn, the remotest planet before known. A century only had elapsed
since the theory of gravitation had been established. Yet it was at
once perceived how greatly this theory had increased the power of the
astronomer to deal with planetary motions. Before a year had passed
more was known about the motions of Uranus than had been learned about
the motion of any of the old planets during the two thousand years
preceding the time of Copernicus. It was possible to calculate in
advance the position of the newly discovered planet, to calculate
retrogressively the path along which it had been travelling, unseen
and unsuspected, during the century preceding its discovery. And now
observations which many might have judged to be of little value, came
in most usefully. Astronomers since the discovery of the telescope had
formed catalogues of the places of many hundreds of stars invisible to
the naked eye. Search among the observations by which such catalogues
had been formed, revealed the fact that Uranus had been seen and
catalogued as a fixed star twenty-one several times! Flamsteed had seen
it five times, each time recording it as a star of the sixth magnitude,
so that five of Flamsteed's stars had to be cancelled from his lists.
Lemonnier had actually seen Uranus twelve times, and only escaped the
honour of discovering the planet (as such) through the most marvellous
carelessness, his astronomical papers being, as Arago said, 'a very
picture of chaos.' Bradley saw Uranus three times.[3] Mayer saw the
planet once only.
It was from the study of the movements of Uranus as thus seen, combined
with the planet's progress after its discovery, that mathematicians
first began to suspect the existence of some unknown disturbing body.
The observations preceding the discovery of the planet range over an
interval of ninety years and a few months, the earliest observation
used being one made by Flamsteed on December 23, 1690. There is
something very strange in the thought that science was able thus to
deal with the motions of a planet for nearly a century before the
planet was known. Astronomy calculated in the first place where the
planet had been during that time; and then, from records made by
departed observers, who had had no suspicion of the real nature of the
body they were observing, Astronomy corrected her calculations, and
deduced more rigorously the true nature of the new planet's motions.
But still stranger and more impressive is the thought that from
researches such as these, Astronomy should be able to infer the
existence of a planet a thousand million miles further away than Uranus
itself. How amazing it would have seemed to Flamsteed, for example, if
on that winter evening in 1693, when he first observed Uranus, he had
been told that the orb which he was entering in his lists as a star of
the sixth magnitude was not a star at all, and that the observation he
was then making would help astronomers a century and a half later to
discover an orb a hundred times larger than the earth, and travelling
thirty times farther away from the sun.
Even more surprising, however, than any of the incidents which preceded
the discovery of Neptune was this achievement itself. That a planet so
remote as to be quite invisible to the naked eye, never approaching
our own earth within less than twenty-six hundred millions of miles,
never even approaching Uranus within less than nine hundred and fifty
millions of miles, should be detected by means of those particular
perturbations (among many others) which it produced upon a planet not
yet known for three-quarters of a century, seemed indeed surprising.
Yet even this was not all. As if to turn a wonderful achievement into
a miracle of combined skill and good fortune, came the announcement
that, after all, the planet discovered in the spot to which Adams
and Leverrier pointed was not the planet of their calculations, but
travelled in an orbit four or five hundred millions of miles nearer to
the sun than the orbit which had been assigned to the unknown body.
Many were led to suppose that nothing but a most marvellous accident
had rewarded with such singular success the calculations of Adams
and Leverrier. Others were even more surprised to learn that the new
planet departed strangely from the law of distances which all the other
planets of the solar system seemed to obey. For according to that law
(called Bode's law) the distance of Neptune, instead of being about
thirty times, should have been thirty-nine times the earth's distance
from the sun.
In some respects the discovery of a planet nearer to the sun than
Mercury may seem to many far inferior in interest to the detection of
the remote giant Neptune. Between Mercury and the sun there intervenes
a mean distance of only thirty-six millions of miles, a distance
seeming quite insignificant beside those which have been dealt with
in describing the discovery of Uranus and Neptune. Again it is quite
certain that any planet between Mercury and the sun must be far
inferior to our own earth in size and mass, whereas Neptune exceeds
the earth 105 times in size and 17 times in mass. Thus a much smaller
region has to be searched over for a much smaller body. Moreover,
while mathematical calculation cannot deal nearly so exactly with an
intra-Mercurial planet as with Neptune, for there are no perturbations
of Mercury which give the slightest information as to the orbital
position of his disturber, the part of the heavens occupied by the
intra-Mercurial planet is known without calculation, seeing that the
planet must always lie within six or seven degrees or so of the sun,
and can never be very far from the ecliptic.
Yet in reality the detection of an intra-Mercurial planet is a problem
of far greater difficulty than that of such a planet as Neptune, while
even now when most astronomers consider that an intra-Mercurial planet
has been detected, the determination of its orbit is a problem which
seems to present almost insuperable difficulties.