SCHWABE AND THE SUN-SPOT PERIOD
[Sidenote: Discoveries contrary to expectation.]
In preceding chapters we have reviewed discoveries, some of which have
been made as a result of a deliberate search, and others accidentally in
the course of work directed to a totally different end; but so far we have
not considered a case in which the discoverer entered upon an enterprise
from which he was positively dissuaded.
[Sidenote: Nothing expected from spots.]
In the next chapter we shall come across a very striking instance of this
type; but even in the discovery that there was a periodicity in the solar
spots, with which I propose to deal now, Herr Schwabe began his work in
the face of deterrent opinions from eminent men. His definite announcement
was first made in 1843, though he had himself been convinced some years
earlier. In 1857 the Royal Astronomical Society awarded him their gold
medal for the discovery; and in the address delivered on the occasion the
President commenced by drawing attention to this very fact, that
astronomers who had expressed any opinions on the subject had been
uniformly and decidedly against the likelihood of there being anything
profitable in the study of the solar spots. I will quote the exact words
of the President, Mr. Manuel Johnson, then Radcliffe Observer at Oxford.
"It was in 1826 that Heinrich Schwabe, a gentleman resident in
Dessau, entered upon those researches which are now to engage our
attention. I am not aware of the motive that induced him--whether any
particular views had suggested themselves to his own mind--or whether
it was a general desire of investigating, more thoroughly than his
predecessors had done, the laws of a remarkable phenomenon, which it
had long been the fashion to neglect. He could hardly have
anticipated the kind of result at which he has arrived; at the same
time we cannot imagine a course of proceeding better calculated for
its detection, even if his mind had been prepared for it, than that
which he has pursued from the very commencement of his career.
Assuredly if he entertained such an idea, it was not borrowed from
the authorities of the last century, to whom the solar spots were
objects of more attention than they have been of late years.
"'Nulla constanti temporum lege apparent aut evanescunt,' says Keill
in 1739.--_Introduct. ad Physic. Astronom._, p. 253.
"'Il est manifest par ce que nous venons de rapporter qu'il n'y a
point de regle certaine de leur formation, ni de leur nombre et de
leur figure,' says Cassini II. in 1740.--_Elem d'Astron._, vol. i. p.
82.
"'Il semble qu'elles ne suivent aucune loi dans leur apparitions,'
says Le Monnier in 1746.--_Instit. Astron._, p. 83.
"'Solar spots observe no regularity in their shape, magnitude,
number, or in the time of their appearance or continuance,' says Long
in 1764.--_Astron._, vol. ii. p. 472.
"'Les apparitions des taches du soleil n'ont rien de regulier,' says
Lalande in 1771.--_Astron._, vol. iii. Sec. 3131, 2nd edit.
"And Delambre's opinion may be inferred from a well-known passage in
the third volume of his 'Astronomy' (p. 20), published in 1814, where
treating of the solar spots he says, 'Il est vrai qu'elles sont plus
curieuses que vraiment utiles.'"[4]
It will thus be evident that Herr Schwabe had the courage to enter upon a
line of investigation which others had practically condemned as likely to
lead nowhere, and that his discovery was quite contrary to expectation. It
is a lesson to us that not even the most unlikely line of work is to be
despised; for the outcome of Schwabe's work was the first step in the
whole series of discoveries which have gradually built up the modern
science of Solar Physics, which occupies so deservedly large a part of the
energies of, for instance, the great observatory attached to the
University of Chicago.
[Sidenote: Schwabe's announcement.]
It has been our practice to recall the actual words in which the
discoverer himself stated his discovery, and I will give the original
modest announcement of Schwabe, though for convenience of those who do not
read German I will attempt a rough translation. He had communicated year
by year the results of his daily counting of the solar spots to the
_Astronomische Nachrichten_, and after he had given ten years' results in
this way he collected them together, but he made no remark on the curious
sequence which they undoubtedly showed at that time. Waiting patiently six
years for further material, in 1843 he ventured to make his definite
announcement as follows:--"From my earlier observations, which I have
communicated annually to this journal, there was manifest already a
certain periodicity of sun-spots; and the probability of this being really
the case is confirmed by this year's results. Although I gave in volume 15
the total numbers of groups for the years 1826-1837, nevertheless I will
repeat here a complete series of all my observations of sun-spots, giving
not only the number of groups, but also the number of days of observation,
and further the days when the sun was free from spots. The number of
groups alone will not in itself give sufficient accuracy for determination
of a period, since I have convinced myself that when there are a large
number of sun-spots the number will be reckoned somewhat too small, and
when few sun-spots, the number somewhat too large; in the first case
several groups are often counted together in one, and in the second it is
easy to divide a group made up of two component parts into two separate
groups. This must be my excuse for repeating the early catalogue, as
follows:--
+---------------------------------------------+
| Year.| Number of | Days free | Days of |
| | Groups. | from Spots.| Observation.|
|---------------------------------------------|
| 1826 | 118 | 22 | 277 |
| 1827 | 161 | 2 | 273 |
| 1828 | 225 | 0 | 282 |
| 1829 | 199 | 0 | 244 |
| 1830 | 190 | 1 | 217 |
|---------------------------------------------|
| 1831 | 149 | 3 | 239 |
| 1832 | 84 | 49 | 270 |
| 1833 | 33 | 139 | 267 |
| 1834 | 51 | 120 | 273 |
| 1835 | 173 | 18 | 244 |
|---------------------------------------------|
| 1836 | 272 | 0 | 200 |
| 1837 | 333 | 0 | 168 |
| 1838 | 282 | 0 | 202 |
| 1839 | 162 | 0 | 205 |
| 1840 | 152 | 3 | 263 |
|---------------------------------------------|
| 1841 | 102 | 15 | 283 |
| 1842 | 68 | 64 | 307 |
| 1843 | 34 | 149 | 324 |
|(1844)| (52) | (111) | (320) |
+---------------------------------------------+
"If we now compare together the number of groups, and the days free from
spots, we find that the sun-spots have a period of about ten years, and
that for about five years they are so numerous that during this period few
days, if any, are free from spots. The sequel must show whether this
period is constant, whether the minimum activity of the sun in producing
spots lasts for one or two years, and whether this activity increases more
quickly than it decreases."
[Illustration:
FEB. 18, 1894. FEB. 19, 1894.
X.--PHOTOGRAPHS OF THE SUN TAKEN AT THE ROYAL OBSERVATORY, GREENWICH,
SHEWING SUNSPOTS.]
[Sidenote: Attracted little attention, until eight years later.]
This brief announcement is all that the discoverer says upon the subject;
and it is perhaps not remarkable that it attracted very little attention,
especially when we remember that it related to a matter which the
astronomical world had agreed to put aside as unprofitable and not worth
attention. Next year, in giving his usual paper on the spots for 1844 he
recurs to the subject in the following sentence: "The periodicity of spots
of about ten years which was indicated in my summary published last year,
is confirmed by this year's observations." I have added in brackets to the
table above reproduced the numbers for 1844 subsequently given, and it
will be seen how nearly they might have been predicted.
[Sidenote: Other phenomena sympathetic and others not.]
Still the subject attracted little attention. Turning over the leaves of
the journal at random, I came across the annual report of the Astronomer
Royal of England, printed at length. But in it there is no reference to
this discovery, which opened up a line of work now strongly represented in
the annual programme of the Royal Observatory at Greenwich. Mr. Johnson
remarks that the only person who had taken it up was Julius Schmidt, who
then resided near Hamburg. But Schwabe went on patiently accumulating
facts; and in 1851 the great Von Humboldt in the third volume of his
_Cosmos_, drew attention to the discovery, which was accordingly for the
first time brought into general notice. It will be seen that there are not
many facts of general interest relating to the actual discovery beyond the
courage with which the work was commenced in a totally unpromising
direction, and the scant attention it received after being made for us. We
may admit that interest centres chiefly in the tremendous consequences
which flowed from it. We now recognise that many other phenomena are bound
up with this waxing and waning of the solar spots. We might be prepared
for a sympathy in phenomena obviously connected with the sun itself; but
it was an unexpected and startling discovery that magnetic phenomena on
the earth had also a sympathetic relation with the changes in sun-spots,
and it is perhaps not surprising that when once this connection of solar
and terrestrial phenomena was realised, various attempts have been made to
extend it into regions where we cannot as yet allow that it has earned a
legitimate right of entry. We have heard of the weather and of Indian
famines occurring in cycles identical with the sun-spot cycle; and it is
obvious how tremendously important it would be for us if this were found
to be actually the case. For we might in this way predict years of
possible famine and guard against them; or if we could even partially
foretell the kind of weather likely to occur some years hence, we might
take agricultural measures accordingly. The importance of the connection,
if only it could be established, is no doubt the reason which has misled
investigators into laying undue stress on evidence which will not bear
close scrutiny. For the present we must say decidedly that no case has
been made out for paying serious attention to the influence of sun-spots
on weather. Nevertheless, putting all this aside, there is quite enough of
first-rate importance in the sequel to Schwabe's discovery.
[Sidenote: Greenwich sun records.]
[Sidenote: The sun's rotation.]
Let us review the facts in order. Most of us, though we may not have had
the advantage of seeing an actual sun-spot through a telescope, have seen
drawings or photographs of spots. There is a famous drawing made by James
Nasmyth (of steam-hammer fame), in July, 1864, which is of particular
interest, because at that time Nasmyth was convinced--and he convinced
many others with him--that the solar surface was made up of a
miscellaneous heap of solid bodies in shape like willow leaves, or grains
of rice, thrown together almost at random, and the drawing was made by him
to illustrate this idea. Comparing a modern photograph with it, we see
that there is something to be said for Nasmyth's view, which attracted
much attention at the time and occasioned a somewhat heated controversy.
But since the invention of the spectroscope it has become quite obsolete;
it probably does not correspond in any way to the real facts. But instead
of looking at pictures which have been enlarged to show the detailed
structure in and near a spot, we will look at a series of pictures of
the whole sun taken on successive days at Greenwich in which the spots are
necessarily much smaller, but which show the behaviour of the spots from
day to day. (See Plates X. and XI.) From the date at the foot of each it
will be seen that they gradually cross the disc of the sun (a fact first
discovered by Galileo in 1610), showing that the sun rotates on an axis
once in about every twenty-five days. There are many interesting facts
connected with this rotation; especially that the sun does not rotate as a
solid body, the parts near the (Sun's) Equator flowing quicker than those
nearer the Poles; but for the present we cannot stop to dwell upon them.
What interests us particularly is the history, not from day to day, but
from year to year, as Schwabe has already given it for a series of years.
[Illustration:
FEB. 20, 1894. FEB. 21, 1894.
XI.--PHOTOGRAPHS OF THE SUN TAKEN AT THE ROYAL OBSERVATORY, GREENWICH,
SHEWING SUNSPOTS]
[Sidenote: Wolf's numbers.]
[Sidenote: Greenwich areas.]
[Sidenote: Magnetic fluctuations.]
When it became generally established that this periodicity existed, Rudolf
Wolf of Zurich collected the facts about sun-spots from the earliest
possible date, and represented this history by a series of numbers which
are still called Wolf's Sun-Spot Numbers. You will see from the diagram
the obvious rise and fall for eleven years,--not ten years, as Schwabe
thought, but just a little over eleven years. The facts are, however,
given more completely by the work done at the Royal Observatory at
Greenwich. It is part of the regular daily work of that Observatory to
photograph the sun at least twice. Many days are of course cloudy or wet,
so that photographs cannot be obtained; but there are available
photographs similarly taken in India or in Mauritius, where the weather is
more favourable, and from these the gaps are so well filled up that very
few days, if any, during the whole year are left without some photograph
of the sun's surface. On these photographs the positions and the areas of
the spots are carefully measured under a microscope, and the results when
submitted to certain necessary calculations are published year by year. It
is clearly a more accurate estimate of the spottedness of the sun to take
the total _area_ of all the spots rather than their mere _number_, for in
the latter case a large spot and a small one count equally. Hence the
Greenwich records will perhaps give us an even better idea of the
periodicity than Wolf's numbers. Now, at the same observatory magnetic
observations are also made continuously. If a magnet be suspended freely
we are accustomed to say that it will point to the North Pole; but this is
only very roughly true. In the first place, it is probably well known to
you that there is a considerable deviation from due north owing to the
fact that the magnetic North Pole is not the same as the geographical
North Pole; but this for the present need not concern us. What does
concern us is, that if the needle is hung up and left long enough to come
to rest, it does not then remain steadily at rest, but executes slow and
small oscillations backwards and forwards, up and down, throughout the
day; repeating nearly the same oscillations on the following day, but at
the same time gradually changing its behaviour so as to oscillate
differently in summer and winter. These changes are very small, and would
pass unnoticed by the naked eye; but when carefully watched through a
telescope, or better still, when photographed by some apparatus which will
at the same time magnify them, they can be rendered easily visible. When
the history of these changes is traced it is seen at once that there is a
manifest connection with the cycle of sun-spot changes; for instance, if
we measure the range of swing backwards and forwards during the day and
take the average for all the days in the year, and then compare this with
the average number of sun-spots, we shall see that the averages rise and
fall together. Similarly we may take the up and down swing, find the
average amount of it throughout the year, and again we shall find that
this corresponds very closely with the average number of sun-spots.
[Illustration: PLATE XII. NUMBER OF SUNSPOTS (Wolf) Compared with DAILY
RANGE of MAGNETIC DECLINATION & DAILY RANGE of MAGNETIC HORZL. FORCE
(as observed at Greenwich.)]
[Sidenote: Daily curves.]
[Sidenote: Difference between summer and winter, and between sun-spot
maximum and minimum.]
[Sidenote: Cause unknown.]
But perhaps the most striking way to exhibit the sympathy is to combine
different variations of the needle into one picture. And first we must
remark that there is another important variation of the earth's magnetic
action which we have not yet considered. We have mentioned the swing of
the needle to and fro, and the swing up and down, and these correspond to
changes in the _direction_ of the force of attraction on the needle. But
there may be also changes in _intensity_ of this action; the pull may be a
little stronger or a little weaker than before, and these variations are
not represented by any actual movement of the needle, though they can be
measured by proper experiments. We can, however, imagine them represented
by a movement of the end of the needle if we suppose it made of elastic
material, so that it would lengthen when the force was greater and
contract slightly when the force was less. If a pencil were attached to
the end of such an elastic needle so as to make a mark on a sheet of
paper, and if for a moment we exclude the up and down movements, the
pencil would describe during the day a curve on the paper, as the end of
the needle swung backwards and forwards with the change in direction, and
moved across the direction of swing with the change in intensity. Now when
curves of this kind are described for a day in each month of the year,
there is a striking difference between the forms of them. During the
summer months they are, generally speaking, comparatively large and open,
and during the winter months they are small and close. This change in form
is seen by a glance at Plate XIII., which gives the curves throughout the
whole of one year. Let us now, instead of forming a curve of this kind for
each month, form a single average curve for the whole year; and let us
further do this for a series of years. (Plate XIV.) We see that the curves
change from year to year in a manner very similar to that in which they
change from month to month in any particular year, and the law of change
is such that in years when there are many sun-spots we get a large open
curve similar to those found in the summer, while for years when there are
few sun-spots we get small close curves very like those in the winter.
Hence we have two definite conclusions suggested: firstly, that the
changes of force are sympathetic with the changes in the sun-spots; and
secondly, that times of maximum sun-spots correspond to summer, and times
of minimum to winter. And here I must admit that this is about as far as
we have got at present in the investigation of this relationship. _Why_
the needle behaves in this way we have as yet only the very vaguest ideas;
suggestions of different kinds have certainly been put forward, but none
of them as yet can be said to have much evidence in favour of its being
the true one. For our present purpose, however, it is sufficient to note
that there is this very real connection, and that consequently Schwabe's
sun-spot period may have a very real importance with regard to changes in
our earth itself.
[Illustration:
GREENWICH MAGNETIC CURVES
1859-60
PLATE XIII.
GREENWICH MAGNETIC CURVES FOR APRIL 1841-1860]
[Sidenote: Illustration of spurious connection.]
But I may perhaps repeat the word of caution already uttered against
extending without sufficient evidence this notion of the influence of
sun-spots to other phenomena, such as weather. A simple illustration will
perhaps serve better than a long argument to show both the way in which
mistakes have been made and the way in which they can be seen to be
mistakes. There is at the Royal Observatory at Greenwich an instrument for
noting the direction of the wind, the essential part being an ordinary
wind-vane, the movements of which are automatically recorded on a sheet of
paper. As the wind shifts from north to east the pencil moves in one
direction, and when it shifts back again towards the north the pencil
moves in the reverse way. But sometimes the wind shifts continuously from
north to east, south, west, and back to north again, the vane making a
complete revolution; and this causes the pencil to move continuously in
one direction, until when the vane has come to north again, the pencil is
far away from the convenient place of record; on such occasions it is
often necessary to replace it by hand. Then again, the vane may turn in
the opposite direction, sending the pencil inconveniently to the other
side of the record. During the year it is easy to count the number of
complete changes of wind in either direction, and subtracting one number
from the other, we get the excess of complete revolutions of the vane in
one direction over that in the other. Now if these rather arbitrary
numbers are set down year by year, or plotted in the shape of a diagram,
we get a curve which may be compared with the sun-spot curve, and during a
period of no less than sixteen years--from 1858 to 1874--there was a
remarkable similarity between the two diagrams. From this evidence _alone_
it might fairly be inferred that the sun-spots had some curious effect
upon the weather at Greenwich, traceable in this extraordinary way in the
changes of the wind. But the particular way in which these changes are
recorded is so arbitrary that we should naturally feel surprise if there
was a real connection between the two phenomena; and fortunately there
were other records preceding these years and following them which enabled
us to test the connection further, and it was found, as we might naturally
expect, that it was not confirmed. On looking at diagrams (Plate XV.) for
the periods before and after, no similarity can be traced between the
sun-spot curve and the wind-vane curve, and we infer that the similarity
during the period first mentioned was entirely accidental. This shows that
we must be cautious in accepting, from a limited amount of evidence, a
connection between two phenomena as real and established; for it may be
purely fortuitous. We may particularly remark that it is desirable to have
repetitions through several complete periods instead of one alone. It is
possible to reduce to mathematical laws the rules for caution in this
matter; and much useful work has already been done in this direction by
Professor Schuster of Manchester and others, though as yet too little
attention has been paid to their rules by investigators naturally eager to
discover some hitherto unthought-of connection between phenomena.
[Sidenote: Faculae follow spots and the chromosphere.]
With this example of the need for caution, we may return to phenomena of
which we can certainly say that they vary sympathetically with the
sun-spots. Roughly speaking, the whole history of the sun seems to be
bound up with them. Besides these dark patches which we call spots (which,
by the way, are not really dark but only less bright than the surrounding
part of the disc), there are patches brighter than the rest which have
been called faculae. With ordinary telescopes, either visual or
photographic, these can generally only be detected near the edge of the
sun's disc; but even with this limitation it can easily be established
that the faculae vary in number and size from year to year much in the same
way as the spots, and this conclusion is amply confirmed by the beautiful
method of observing the faculae with the new instrument designed by
Professor Hale of the Yerkes Observatory. With this instrument, called a
spectroheliograph, it is possible to photograph the faculae in all parts of
the sun's disc, and thus to obtain a much more complete history of them,
and there is no doubt whatever of their variation sympathetically with the
spots. Nor is there any doubt about similar variations in other parts of
the sun which we cannot see _at all_ with ordinary telescopes, except on
the occasions when the sun is totally eclipsed. Roughly speaking, these
outlying portions of the sun consist of two kinds, the chromosphere and
the corona, the former looking like an irregular close coating of the
ordinary sun, and the latter like a pearly halo of light extending to
many diameters of the sun's disc, but not with any very regular form.
[Illustration: PLATE XV. SMOOTHED SUNSPOT CURVE (WOLF) COMPARED WITH THE
NUMBER OF TURNS MADE IN EACH YEAR BY THE OSLER ANEMOMETER VANE OF THE
ROYAL OBSERVATORY, GREENWICH (THE EXCESS OF THE DIRECT TURNS (D) OVER THE
RETROGRADE TURNS (R) OR _VICE VERSA_.)
THE UPPER CURVE IS IN EACH CASE THE SUNSPOT CURVE, THE LOWER THE VANE
CURVE. THE BREAK IN 1882 IN THE VANE CURVE IS DUE TO THE OMISSION OF
EVIDENTLY ACCIDENTAL TURNS FROM THAT DATE.]
The chromosphere, from which shoot out the prominences or "red flames,"
can now be observed without an eclipse if we employ the beautiful
instrument above-mentioned, the spectroheliograph; and Professor Hale has
succeeded in photographing spots, faculae, and prominences all on the same
plate. But although many have made the attempt (and Professor Hale,
perhaps, a more determined attempt than any man living), no one has yet
succeeded in obtaining any picture or evidence of the existence of the
corona excepting on the occasion of a total solar eclipse.
[Sidenote: Eclipses of sun.]
[Sidenote: Total eclipses rare.]
Now these occasions are very rare. There are two or three eclipses of the
sun every year, but they are generally of the kind known as partial; when
the moon does indeed come between us and the sun to some extent, but only
cuts off a portion of his light--a clean-cut black disc is seen to
encroach more or less on the surface of the sun. Most of us have had an
opportunity of seeing a partial eclipse, probably more than once; but few
have seen a total eclipse. For this the moon must come with great
exactness centrally between us and the sun; and the spot where this
condition is fulfilled completely only covers a few hundred miles of the
earth's surface at one moment. As the earth turns round, and as the moon
revolves in its orbit, this patch from which the sun is totally eclipsed
travels over the earth's surface, marking out a track some thousands of
miles in length possibly, but still not more than 200 miles wide; and in
order to see the sun totally eclipsed even on the rare occasions when it
is possible at all (for, as already remarked, in the majority of cases the
eclipse is only partial), we must occupy some station in this narrow belt
or track, which often tantalisingly passes over either the ocean or some
regions not easily accessible to civilised man. Moreover, if we travel to
such favoured spots the whole time during which the sun is totally
eclipsed cannot exceed a few minutes, and hence observations are made
under rather hurried and trying conditions. In these modern days of
photography it is easier to take advantage of these precious moments than
it used to be when there was only the eye and memory of an excited
observer to rely upon. It is perhaps not surprising that some of the
evidence collected on these earlier occasions was conflicting; but
nowadays the observers, generally speaking, direct their energies in the
first place to mounting accurately in position photographic apparatus of
different kinds, each item of it specially designed to settle some
particular problem in the most feasible way; secondly, to rehearsing very
carefully the exact programme of exposures necessary during the critical
few minutes; and finally, to securing these photographs with as few
mistakes as possible when the precious moments actually arrive. Even then
the whole of their efforts are quite likely to be rendered unavailing by a
passing cloud; and bitter is the disappointment when, after travelling
thousands of miles, and spending months in preparation, the whole
enterprise ends in nothing owing to some caprice of the weather.
[Sidenote: Corona follows spots.]
Hence it will easily be imagined that our knowledge of the corona, the
part of the sun which we can still only study on occasions of a total
solar eclipse, advances but slowly. During the last twenty years there has
been altogether scarcely half-an-hour available for this research, though
it may fairly be said that the very best possible use has been made of
that half-hour. And, what is of importance for our immediate purpose, it
has gradually been established by comparing the photographs of one eclipse
with those of another, that the corona itself undergoes distinct changes
in form in the same period which governs the changes of sun-spots. When
there are many sun-spots the corona spreads out in all directions from the
edge of the sun's disc; when there are few sun-spots the corona extends
very much further in the direction of the sun's equator, so that at
sun-spot minimum there is an appearance of two huge wings. Although the
evidence is necessarily collected in a scrappy manner, by this time there
is sufficient to remove this relationship out of the region of mere
suspicion, and to give it a well-established place in our knowledge of the
sun's surroundings.
[Sidenote: Corona may influence magnets.]
Now the corona of the sun may be compared to some rare animal which we
only see by paying a visit to some distant land, and may consider
ourselves even then fortunate to get a glimpse of; and it might be thought
that the habits of such an animal are not likely to be of any great
importance in our everyday life. But so far from this being the case in
regard to the corona, it is more than possible that the knowledge of its
changes may be of vital interest to us. I have already said that, as yet,
we have no satisfactory account of the reason why changes in sun-spots
seem to influence changes in our magnets on the earth; but one of the
theories put forward in explanation, and one by no means the least
plausible, is that this influence may come, not from the sun-spots
themselves, but from some other solar phenomenon which varies in sympathy
with them; and in particular that it may come from the corona. These wings
which reach out at sun-spot minimum can be seen to extend a considerable
distance, and there is no reason to suppose that they actually cease at
the point where they become too faint for us to detect them further; they
may extend quite as far as the earth itself and even beyond; and they may
be of such a nature as to influence our magnets. As the earth revolves
round the sun it may sometime plunge into them, to emerge later and pass
above or below them; as again the wings spread themselves at sun-spot
minimum and seem to shrink at maximum, so our magnets may respond by
sympathetic though very small vibrations. Hence it is quite possible that
the corona is directly influencing the magnetic changes on the earth.
[Sidenote: Possible importance of corona.]
But it may be urged that these changes are so slight as to be merely of
scientific interest. That may be true to-day, but who will be bold enough
to say that it will be true to-morrow? If we are thinking of practical
utility alone, we may remember that two great forces of Nature which we
have chained into the service of man, steam and electricity, put forth
originally the most feeble manifestations, which might readily have been
despised as valueless; but by careful attention to proper conditions
results of overwhelming practical importance have been obtained from these
forces, which might have been, and for many centuries were, neglected as
too trivial to be worth attention. Recently the world has been startled by
the discovery of new elements, such as radium, whose very existence was
only detected by a triumph of scientific acuteness in investigation, and
yet which promise to yield influences on our lives which may overwhelm in
importance all that has gone before. And similarly it may be that these
magnetic changes, when properly interpreted or developed, may become of an
importance in the future out of all proportion to the attention which they
have hitherto attracted. Hence, although perhaps sufficient has already
been established to show the immense consequences which flow from
Schwabe's remarkable discovery of the periodicity in solar spots, we may
be as yet only on the threshold of its real value.
From what little causes great events spring! How little can Schwabe have
realised, when he began to point his modest little telescope at the sun,
and to count the number of spots--the despised spots which he had been
assured were of no interest and exhibited no laws, and were generally
unprofitable--that he was taking the first step in the invention of the
great science of Solar Physics!--a science which is, I am glad to say,
occupying at the present moment so much of the attention, not only of the
great Yerkes Observatory, but of many other observatories scattered over
the globe.