LIGHT SCIENCE.
LONDON: PRINTED BY
SPOTTISWOODE AND CO., NEW-STREET SQUARE
AND PARLIAMENT STREET
LIGHT SCIENCE FOR
LEISURE HOURS.
A SERIES OF FAMILIAR ESSAYS
ON
SCIENTIFIC SUBJECTS, NATURAL PHENOMENA, &c.
BY
RICHARD A. PROCTOR,
AUTHOR OF ‘THE SUN’ ‘OTHER WORLDS THAN OURS’ ‘SATURN’ ETC.
‘I bear you witness as ye bear to me,
Time, day, night, sun, stars, life, death, air, sea, earth.’
_Swinburne._
_NEW EDITION._
LONDON:
LONGMANS, GREEN, AND CO.
1886.
PREFACE
TO
THE FIFTH EDITION.
In preparing this edition, only those passages which have been shown
by recent researches to be erroneous have been removed. It has not
been thought necessary, or even desirable, to modify the wording of
Essays (by changes of tense or otherwise) in such a way that, as thus
modified, the Essays might have appeared in 1884. In many cases this
would have been altogether misleading, whereas, with the dates prefixed
to the several Essays, no misconceptions can arise.
RICHARD A. PROCTOR.
_PREFACE TO SECOND EDITION._
This edition has been carefully revised, and, in parts, considerably
modified. Thus the Essay on Britain’s Coal Cellars has been added,
and two Essays on Government Aid to Science have been removed. I
may mention that my views on the subject of the last-named Essays
have changed altogether since those Essays were written—certain
circumstances which have come under my observation having convinced me
that more mischief than advantage would result from any wide scheme for
securing Government aid for scientific researches.
RICHARD A. PROCTOR.
LONDON: _January 1873_.
_PREFACE TO FIRST EDITION._
In preparing these Essays, my chief object has been to present
scientific truths in a light and readable form—clearly and simply,
but with an exact adherence to the facts as I see them. I have
followed—here and always—the rule of trying to explain my meaning
precisely as I should wish others to explain, to myself, matters with
which I was unfamiliar. Hence I have avoided that excessive simplicity
which some seem to consider absolutely essential in scientific essays
intended for general perusal, but which is often even more perplexing
than a too technical style. The chief rule I have followed, in order to
make my descriptions clear, has been to endeavour to make each sentence
bear one meaning, and one only. Speaking as a reader, and especially as
a reader of scientific books, I venture to express an earnest wish that
this simple rule were never infringed, even to meet the requirements of
style.
It will hardly be necessary to mention that several of the shorter
Essays are rather intended to amuse than to instruct.
The Essay on the influence which marriage has been supposed to exert on
the death-rate is the one referred to by Mr. Darwin at page 176 (vol.
i.) of his ‘Descent of Man.’
RICHARD A. PROCTOR.
LONDON: _May_ 1871.
CONTENTS.
PAGE
STRANGE DISCOVERIES RESPECTING THE AURORA 1
THE EARTH A MAGNET 14
OUR CHIEF TIME-PIECE LOSING TIME 30
ENCKE THE ASTRONOMER 46
VENUS ON THE SUN’S FACE 49
BRITAIN’S COAL CELLARS 72
THE SECRET OF THE NORTH POLE 97
IS THE GULF STREAM A MYTH? 114
FLOODS IN SWITZERLAND 133
A GREAT TIDAL WAVE 138
DEEP-SEA DREDGINGS 142
THE TUNNEL THROUGH MONT CENIS 148
TORNADOES 153
VESUVIUS 167
THE EARTHQUAKE IN PERU 189
THE GREATEST SEA-WAVE EVER KNOWN 194
THE USEFULNESS OF EARTHQUAKES 211
THE FORCING POWER OF RAIN 225
A SHOWER OF SNOW-CRYSTALS 230
LONG SHOTS 233
INFLUENCE OF MARRIAGE ON THE DEATH-RATE 238
THE TOPOGRAPHICAL SURVEY OF INDIA 244
A SHIP ATTACKED BY A SWORD-FISH 256
THE SAFETY-LAMP 259
THE DUST WE HAVE TO BREATHE 265
PHOTOGRAPHIC GHOSTS 267
THE OXFORD AND CAMBRIDGE ROWING STYLES 269
BETTING ON HORSE RACES: OR, THE STATE OF THE ODDS 274
SQUARING THE CIRCLE 288
A NEW THEORY OF ACHILLES’ SHIELD 297
LIGHT SCIENCE FOR LEISURE HOURS.
_STRANGE DISCOVERIES RESPECTING THE AURORA._
The brilliant streamers of coloured light which wave at certain
seasons over the heavens have long since been recognised as among the
most singular and impressive of all the phenomena which the skies
present to our view. There is something surpassingly beautiful in
the appearance of the true ‘auroral curtain.’ Fringed with coloured
streamers, it waves to and fro as though shaken by some unseen hand.
Then from end to end there pass a succession of undulations, the folds
of the curtain interwrapping and forming a series of graceful curves.
Suddenly, and as by magic, there succeeds a perfect stillness, as
though the unseen power which had been displaying the varied beauties
of the auroral curtain were resting for a moment. But even while the
motion of the curtain is stilled we see its light mysteriously waxing
and waning. Then, as we gaze, fresh waves of disturbance traverse the
magic canopy. Startling coruscations add splendour to the scene, while
the noble span of the auroral arch, from which the waving curtain
seems to depend, gives a grandeur to the spectacle which no words can
adequately describe. Gradually, however, the celestial fires which have
illuminated the gorgeous arch seem to die out. The luminous zone breaks
up. The scene of the display becomes covered with scattered streaks and
patches of ashen grey light, which hang like clouds over the northern
heavens. Then these in turn disappear, and nothing remains of the
brilliant spectacle but a dark smoke-like segment on the horizon.
Such is the aurora as seen in arctic or antarctic regions, where the
phenomenon appears in its fullest beauty. Even in our own latitudes,
however, strikingly beautiful auroral displays may sometimes be
witnessed. Yet those who have seen the spectacle presented near the
true home of the aurora, recognise in other auroras a want of the
fulness and splendour of colour which form the most striking features
of the arctic and antarctic auroral curtains.
Physicists long since recognised in the aurora a phenomenon of more
than local, of more even than terrestrial, significance. They learned
to associate it with relations which affect the whole planetary scheme.
Let us inquire how this had come about.
So long as men merely studied the appearances presented by the aurora,
so long, in fact, as they merely regarded the phenomenon as a local
display, they could form no adequate conception of its importance. The
circumstance which first revealed something of the true character of
the aurora was one which seemed to promise little.
Arago was engaged in watching from day to day, and from year to year,
the vibrations of the magnetic needle in the Paris Observatory.
He traced the slow progress of the needle to its extreme westerly
variation, and watched its course as it began to retrace its way
towards the true north. He discovered the minute vibration which the
needle makes each day across its mean position. He noticed that this
vibration is variable in extent, and so he was led to watch it more
closely. Thus he had occasion to observe more attentively than had yet
been done the sudden irregularities which occasionally characterise the
daily movements of the needle.
All this seems to have nothing to do with the auroral streamers; but
we now reach the important discovery which rewarded Arago’s patient
watchfulness.
In January 1819 he published a statement to the effect that the sudden
changes of the magnetic needle are often associated with the occurrence
of an aurora. I give the statement in his own words, as translated by
General Sabine:—‘Auroras ought to be placed in the first rank among
the causes which sometimes disturb the regular march of the diurnal
changes of the magnetic needle. These do not, even in summer, exceed a
quarter of a degree, but when an aurora appears, the magnetic needle
is often seen to move in a few instants over several degrees.’ ‘During
an aurora,‘ he adds, ‘one often sees in the northern region of the
heavens luminous streamers of different colours shoot from all points
of the horizon. The point in the sky to which these streamers converge
is precisely the point to which a magnetised needle suspended by its
centre of gravity directs itself.... It has, moreover, been shown
that the concentric circular segments, almost similar in form to the
rainbow, which are usually seen previous to the appearance of the
luminous streamers, have their two extremities resting on two parts
of the horizon which are equally distant from the direction towards
which the needle turns; and the summit of each arc lies exactly in
that direction. _From all this, it appears, incontestably, that there
is an intimate connection between the causes of auroras and those of
terrestrial magnetism._’
This strange hypothesis was, at first, much opposed by scientific men.
Amongst others, the late Sir David Brewster pointed out a variety of
objections, some of which appeared at first sight of great force.
Thus, he remarked that magnetic disturbances of the most remarkable
character have often been observed when no aurora has been visible; and
he noticed certain peculiarities in the auroras observed near the polar
regions, which did not seem to accord with Arago’s view.
But gradually it was found that physicists had mistaken the character
of the auroral display. It appeared that the magnetic needle not
only swayed responsively to auroras observable in the immediate
neighbourhood, but to auroras in progress hundreds or even thousands
of miles away. Nay, as inquiry progressed, it was discovered that
the needles in our northern observatories are swayed by influences
associated even with the occurrence of auroras around the southern
polar regions.
In fact, not only have the difficulties pointed out (very properly, it
need hardly be remarked) by Sir David Brewster been wholly removed; but
it has been found that a much closer bond of sympathy exists between
the magnetised needle and the auroral streamers than even Arago had
supposed. It is not merely the case that while an auroral display is
in progress the needle is subject to unusual disturbance, but the
movements of the needle are actually synchronous with the waving
movements of the mysterious streamers. An aurora may be in progress in
the north of Europe, or even in Asia or America, and as the coloured
banners wave to and fro, the tiny needle, watched by patient observers
at Greenwich or Paris, will respond to every phase of the display.
And I may notice in passing that two very interesting conclusions
follow from this peculiarity. First, every magnetic needle over the
whole earth must be simultaneously disturbed; and secondly, the
auroral streamers which wave across the skies of one country must move
synchronously with those which are visible in the skies of another
country, even though thousands of miles may separate the two regions.
But I must pass on to consider further the circumstances which give
interest and significance to the strange discovery which is the subject
of this paper.
Could we only associate auroras with terrestrial magnetism, we should
still have done much to enhance the interest which the beautiful
phenomenon is calculated to excite. But when once this association has
been established, others of even greater interest are brought into
recognition. For terrestrial magnetism has been clearly shown to be
influenced directly by the action of the sun. The needle in its daily
vibration follows the sun, not indeed through a complete revolution,
but as far as the influence of other forces will permit. This has
been abundantly confirmed, and is a fact of extreme importance in the
theory of terrestrial magnetism. Wherever the sun may be, either on the
visible heavens or on that half of the celestial sphere which is at the
moment beneath the horizon, the end of the needle nearest to the sun
makes an effort (so to speak) to point more directly towards the great
ruling centre of the planetary scheme. Seeing, then, that the daily
vibration of the needle is thus caused, we recognise the fact that the
disturbances of the daily vibration may be referred to some peculiarity
of the solar action.
It was not, therefore, so surprising as many have supposed, that the
increase and diminution of these disturbances, in a period of about
eleven years, should be found to correspond with the increase and
diminution of the number of solar spots in a period of equal length.
We already begin to see, then, that auroras are associated in some
mysterious way with the action of the solar rays. The phenomenon which
had been looked on for so many ages as a mere spectacle, caused perhaps
by some process in the upper regions of the air, of a simply local
character, has been brought into the range of planetary phenomena. As
surely as the brilliant planets which deck the nocturnal skies are
illuminated by the same orb which gives us our days and seasons, so
they are subject to the same mysterious influence which causes the
northern banners to wave resplendently over the star-lit depths of
heaven. Nay, it is even probable that every flicker and coruscation
of our auroral displays corresponds with similar manifestations upon
every planet which travels round the sun. It becomes, then, a question
of exceeding interest to inquire what is the nature of the mysterious
apparition which from time to time illuminates our skies. We have
learnt something of the laws according to which the aurora appears; but
what is its true nature? What sort of light is that which illuminates
the heavens? Is there some process of combustion going on in the upper
regions of our atmosphere? Or are the auroral streamers electric or
phosphorescent? Or, lastly, is the light simply solar light reflected
from some substance which exists at an enormous elevation above the
earth?
All these views have from time to time found supporters among
scientific men. It need hardly be said that what we now know of the
association between auroral action and some form of solar disturbance,
would at once enable us to reject some of these hypotheses. But we need
not discuss the subject from this point of view, because a mode of
research has recently been rendered available which at once answers our
inquiries as to the general character of any kind of light. I proceed
to consider the application of this method to the light from the
auroral streamers.
The spectroscope, or, as we may term the instrument, the
‘light-sifter,’ tells us of what nature an object which is a source
of light may be. If the object is a luminous solid or liquid, the
instrument converts its light into a rainbow-coloured streak. If the
object is a luminous vapour, its light is converted into a few bright
lines. And lastly, if the object is a luminous solid or liquid shining
through any vapours, the rainbow-coloured streak again makes its
appearance, but it is now crossed by dark lines, corresponding to the
vapours which surround the object and absorb a portion of its light.
But I must not omit to notice two circumstances which render the
interpretation of a spectrum somewhat less simple than it would
otherwise be.
In the first place, if an object is shining by reflected light its
spectrum is precisely similar to that of the object whose light
illuminates it. Thus we cannot pronounce positively as to the nature
of an object merely from the appearance of its spectrum, unless we
are quite certain that the object is self-luminous. For example, we
observe the solar spectrum to be a rainbow-coloured streak crossed by
a multitude of dark lines, and we conclude accordingly that the sun is
an incandescent globe shining through a complex vaporous atmosphere.
We feel no doubt on this point, because we are absolutely certain
that the sun is self-luminous. Again, we observe the spectrum of the
moon to be exactly similar to the solar spectrum, only, of course,
much less brilliant. And here also we feel no doubt in interpreting
the result. We know, certainly, that the moon is not self-luminous,
and therefore we conclude with the utmost certainty that the light
we receive from her is simply reflected solar light. So far all is
clear. But now take the case of an object like a comet, which may or
may not be self-luminous. If we find that a comet’s spectrum resembles
the sun’s—and this is not altogether a hypothetical case, for a
portion of the light of every comet yet examined does in reality give
a rainbow-coloured streak resembling the solar spectrum—we cannot
form, in that case, any such positive conclusion. The comet may be a
self-luminous body; but, on the other hand, its light may be due merely
to the reflection of the solar beams. Accordingly, the spectroscopist
always accompanies the record of such an observation with an expression
of doubt as to the real nature of the object which is the source of
light.
Secondly, when an electric spark flashes through any vapour, its light
gives a spectrum which indicates the nature, not only of the vapour
through which the spark has passed, but of the substances between
which the spark has travelled. Thus, if we cause an electric flash to
pass between iron points through common air, we see in the spectrum
the numerous bright lines which form the spectrum of iron, and in
addition we see the bright lines belonging to the gases which form our
atmosphere.
Both the considerations above discussed are of the utmost importance
in studying the subject of the auroral light as analysed by the
spectroscope, because there are many difficulties in forming a
general opinion as to the nature of the auroral light, while there
are circumstances which would lead us to anticipate that the light is
electric.
I notice also in passing that we owe to the Swedish physicist Ångström
a large share of the researches on which the above results respecting
the spectrum of the electric spark are founded. The reader will
presently see why I have brought Ångström’s name prominently forward in
connection with the interesting branch of spectroscopic analysis just
referred to. If the discovery we are approaching had been effected by a
tyro in the use of the spectroscope, doubts might very reasonably have
been entertained respecting the exactness of the observations on which
the discovery rests.
It was suggested many years ago, long indeed before the true powers of
spectroscopic analysis had been revealed, that perhaps if the light
of the aurora were analysed by the prism, evidence could be obtained
of its electric nature. The eminent meteorologist Dové remarked, for
instance, that ‘the peculiarities presented by the electric light
are so marked that it appears easy to decide definitely by prismatic
analysis whether the light of the aurora is or is not electric.’
Singularly enough, however, the first proof that the auroral light
is of an electric nature was derived from a very different mode of
inquiry. Dr. Robinson, of Armagh, discovered in 1858 (a year before
Kirchhoff’s recognition of the powers of spectroscopic analysis) that
the light of the aurora possesses in a peculiar degree a property
termed fluorescence, which is a recognised and characteristic property
of the light produced by electrical discharges. ‘These effects,’ he
remarks of the appearances presented by the auroral light under the
tests he applied, ‘were so strong in relation to the actual intensity
of the light, that they appear to afford an additional evidence of the
electric origin of the phenomenon.’
Passing over this ingenious application of one of the most singular
and interesting properties of light, we find that the earliest
determination of the real nature of the auroral light—or rather of its
spectrum—was that effected by Ångström. This observer took advantage
of the occurrence of a brilliant aurora in the winter of 1867-68 to
analyse the spectrum of the coloured streamers. _A single bright line
only was seen!_ Otto Struve, an eminent Russian astronomer, shortly
afterwards made confirmatory observations. At the meeting of the Royal
Astronomical Society in June, 1868, Mr. Huggins thus described Struve’s
results:—‘In a letter, M. Otto Struve has informed me that he has
had two good opportunities of observing the spectrum of the aurora
borealis. The spectrum consists of one line, and the light is therefore
monochromatic. The line falls near the margin of the yellow and green
portions of the spectrum.... This shows that the monochromatic light is
greenish, which surprised me; but General Sabine tells me that in his
polar expeditions he has frequently seen the aurora tinged with green,
and this appearance corresponds with the position of the line seen by
M. Struve.’
The general import of this observation there is no mistaking. It
teaches us that the light of the aurora is due to luminous vapour,
and we may conclude, with every appearance of probability, that the
luminosity of the vapour is due to the passage of electric discharges
through it. It is, however, possible that the position of the bright
line may be due to the character of the particles between which the
discharges take place.
But the view we are to take must depend upon the position of the line.
Here a difficulty presents itself. There is no known terrestrial
element whose spectrum has a bright line precisely in the position
of the line in the auroral spectrum. And mere proximity has no
significance whatever in spectroscopic analysis. Two elements differing
as much from each other in character as iron and hydrogen may have
lines so closely approximating in position that only the most powerful
spectroscope can indicate the difference. So that when Ångström
remarks that the bright line he has seen lies slightly to the left
of a well-known group of lines belonging to the metal calcium (the
principal ingredient of common chalk), we are by no means to infer
that he supposes the substance which causes the presence of the bright
line has any resemblance to that element. Until we can find an element
which has a bright line in its spectrum absolutely coincident with the
bright line detected by Ångström in the spectrum of the aurora,[1] all
speculation as to the real nature of the vapour in which the auroral
electric discharge takes place, or of the substances between which the
spark travels, is altogether precluded.
It was supposed after the total solar eclipse of 1869 that the spectrum
of the sun’s corona exhibited the same bright lines as the aurora. But
recent observations show that the coincidence is not so close as had
been supposed, and, in fact, there is no evidence to show that the
lines are the same.
(From _Fraser’s Magazine_, February 1870.)
_THE EARTH A MAGNET._
There is a very prevalent but erroneous opinion that the magnetic
needle points to the north. I remember well how I discovered in
my boyhood that the needle does _not_ point to the north, for the
discovery was impressed upon me in a very unpleasant manner. I
had purchased a pocket-compass, and was very anxious—not, indeed,
to test the instrument, since I placed implicit reliance upon its
indications—but to make use of it as a guide across unknown regions.
Not many miles from where I lived lay Cobham Wood, no very extensive
forest certainly, but large enough to lose oneself in. Thither,
accordingly, I proceeded with three schoolfellows. When we had lost
ourselves, we gleefully called the compass into action, and made
from the wood in a direction which we supposed would lead us home.
We travelled on with full confidence in our pocket guide; at each
turning we consulted it in an artistic manner, carefully poising it and
waiting till its vibrations ceased. But when we had travelled some two
or three miles without seeing any house or road that we recognised,
matters assumed a less cheerful aspect. We were unwilling to compromise
our dignity as ‘explorers’ by asking the way—a proceeding which no
precedent in the history of our favourite travellers allowed us to
think of. But evening came on, and with it a summer thunder-storm. We
were getting thoroughly tired out, and the _hæc olim meminisse juvabit_
with which we had been comforting ourselves began to lose its force.
When at length we yielded, we learned that we had gone many miles out
of our road, and we did not reach home till several hours after dark.
Also the offending compass was confiscated by justly indignant parents,
so that for a long while the cause of our troubles was a mystery to us.
In reality, instead of pointing due north, the compass pointed more
than 20° towards the west, or nearly to the quarter called by sailors
north-north-west. No wonder, therefore, that we went astray when we
followed a guide so untrustworthy.
The peculiarity that the magnet needle does not, in general, point
to the north, is the first of a series of peculiarities which I now
propose briefly to describe. The irregularity is called by sailors the
needle’s _variation_, but the term more commonly used by scientific
men is the _declination_ of the needle. It was probably discovered
a long time ago, for 800 years before our era the Chinese applied
the magnet’s directive force to guide them in journeying over the
great Asiatic plains, and they must soon have detected so marked a
peculiarity. Instead of a ship’s compass, they made use of a magnetic
car, on the front of which a floating needle carried a small figure,
whose outstretched arm pointed southwards. We have no record, however,
of their discovery of the declination, and know only that they were
acquainted with it in the twelfth century. The declination was
discovered, independently, by European observers in the thirteenth
century.
As we travel from place to place, the declination of the needle is
found to vary. Christopher Columbus was the first to detect this. He
discovered it on the 13th of September, 1492, during his first voyage,
and when he was six hundred miles from Ferro, the most westerly of the
Canary Islands. He found that the declination, which was towards the
east in Europe, passed to the west, and increased continually as he
travelled westwards.
But here we see the first trace of a yet more singular peculiarity.
I have said that at present the declination is towards the west in
Europe. In Columbus’s time it was towards the east. Thus we learn that
the declination varies with the progress of time, as well as with
change of place.
The genius of modern science is a weighing and a measuring one. Men are
not satisfied nowadays with knowing that a peculiarity exists; they
seek to determine its extent, how far it is variable—whether from time
to time or from place to place, and so on. Now the results of such
inquiries applied to the magnetic declination have proved exceedingly
interesting.
We find, first, that the world may be divided into two unequal
portions, over one of which the needle has a westerly, and over the
other an easterly, declination. Along the boundary line, of course, the
needle points due north. England is situated in the region of westerly
magnets. This region includes all Europe, except the north-eastern
parts of Russia; Turkey, Arabia, and the whole of Africa; the greater
part of the Indian Ocean, and the western parts of Australia; nearly
the whole of the Atlantic Ocean; Greenland, the eastern parts of
Canada, and a small slice from the north-eastern part of Brazil. All
these form one region of westerly declination; but, singularly enough,
there lies in the very heart of the remaining and larger region of
easterly magnets an oval space of a contrary character. This space
includes the Japanese Islands, Manchouria, and the eastern parts of
China. It is very noteworthy also, that in the westerly region the
declination is much greater than in the easterly. Over the whole
of Asia, for instance, the needle points almost due north. On the
contrary, in the north of Greenland and of Baffin’s Bay, the magnetic
needle points due west; while still further to the north (a little
westerly), we find the needle pointing with its north end directly
towards the south.
In the presence of these peculiarities, it would be pleasant to
speculate. We might imagine the existence of powerfully magnetic
_veins_ in the earth’s solid mass, coercing the magnetic needle from a
full obedience to the true polar summons. Or the comparative effects of
oceans and of continents might be called into play. But unfortunately
for all this, we have to reconcile views founded on _fixed_ relations
presented by the earth with the process of _change_ indicated above.
Let us consider the declination in England alone.
In the fifteenth century there was an easterly declination. This
gradually diminished, so that in about the year 1657 the needle
pointed due north. After this the needle pointed towards the west,
and continually more and more, so that scientific men, having had
experience only of a continual shifting of the needle in one direction,
began to form the opinion that this change would continue, so that
the needle would pass, through north-west and west, to the south. In
fact, it was imagined that the motion of the needle would resemble
that of the hands of a watch, only in a reversed direction. But before
long observant men detected a gradual diminution in the needle’s
westerly motion. Arago, the distinguished French astronomer and
physicist, was the first (I believe) to point out that ‘the progressive
movement of the magnetic needle towards the west appeared to have
become continually slower of late years’ (he wrote in 1814), ‘which
seemed to indicate that after some little time longer it might become
retrograde.’ Three years later, namely, on the 10th of February,
1817, Arago asserted definitively that the retrograde movement of the
magnetic needle had commenced to be perceptible. Colonel Beaufoy at
first oppugned Arago’s conclusion, for he found from observations made
in London, during the years 1817-1819, that the westerly motion still
continued. But he had omitted to take notice of the circumstance,
that London and Paris are two different places. A few years later
the retrograde motion became perceptible at London also, and it has
now been established by the observations of forty years. It appears,
from a careful comparison of Beaufoy’s observations, that the needle
reached the limit of its western digression (at Greenwich) in March
1819, at which time the declination was very nearly 25°. In Paris, on
the contrary, the needle had reached its greatest western digression
(about 22½°) in 1814. It is rather singular that although at Paris
the retrograde motion thus presented itself five years earlier than in
London, the needle pointed due north at Paris six years later than in
London, viz., in 1663. Perhaps the greater amplitude of the needle’s
London digression may explain this peculiarity.
‘It was already sufficiently difficult,’ says Arago, ‘to imagine
what could be the kind of change in the constitution of the globe
which could act during one hundred and fifty-three years in gradually
transferring the direction of the magnetic needle from due north to 23°
west of north. We see that it is now necessary to explain, moreover,
how it has happened that this gradual change has ceased, and has given
place to a return towards the preceding state of the globe.‘ ‘How is
it,’ he pertinently asks, ‘that the directive action of the globe,
which clearly must result from the action of molecules of which the
globe is composed, can be thus variable, while the number, position,
and temperature of these molecules, and, as far as we know, all their
other physical properties, remain constant?’
But we have considered only a single region of the earth’s surface.
Arago’s opinion will seem still juster when we examine the change which
has taken place in what we may term the ‘magnetic aspect’ of the
whole globe. The line which separates the region of westerly magnets
from the region of easterly magnets now runs, as we have said, across
Canada and eastern Brazil in one hemisphere, and across Russia, Asiatic
Turkey, the Indian Ocean, and West Australia in the other, besides
having an outlying oval to the east of the Asiatic continent. These
lines have swept round a part of the globe’s circuit in a most singular
manner since 1600. They have varied alike in direction and complexity.
The Siberian oval, now distinct, was in 1787 merely a loop of the
eastern line of no declination. The oval appears now to be continually
diminishing, and will one day probably disappear.
We find here presented to us a phenomenon as mysterious, as
astonishing, and as worthy of careful study as any embraced in the wide
domains of science. But other peculiarities await our notice.
If a magnetic needle of suitable length be carefully poised on a fine
point,—or better, be suspended from a silk thread without torsion,—it
will be found to exhibit each day two small but clearly perceptible
oscillations. M. Arago, from a careful series of observations, deduced
the following results:—
At about eleven at night, the north end of the needle begins to move
from west to east, and having reached its greatest easterly excursion
at about a quarter-past eight in the morning, returns towards the west
to attain its greatest westerly excursion at a quarter-past one. It
then moves again to the east, and having reached its greatest easterly
excursion at half-past eight in the evening, returns to the west, and
attains its greatest westerly excursion at eleven, as at starting.
Of course, these excursions take place on either side of the mean
position of the needle, and as the excursions are small, never
exceeding the fifth part of a degree, while the mean position of
the needle lies some 20° to the west of north, it is clear that the
excursions are only nominally eastern and western, the needle pointing
throughout, far to the west.
Now, if we remember that the north end of the needle is that farthest
from the sun, it will be easy to trace in M. Arago’s results a sort of
effort on the part of the needle to turn towards the sun—not merely
when that luminary is above the horizon, but during his nocturnal path
also.
We are prepared, therefore, to expect that a variation having an annual
period, shall appear, on a close observation of our suspended needle.
Such a variation has been long since recognised. It is found that in
the summer of both hemispheres, the daily variation is exaggerated,
while in winter it is diminished.
But besides the divergence of a magnetised needle from the north pole,
there is a divergence from the horizontal position which must now claim
our attention. If a non-magnetic needle be carefully suspended so as to
rest horizontally, and be then magnetised, it will be found no longer
to preserve that position. The northern end _dips_ very sensibly.
This happens in our hemisphere. In the southern, it is the southern
end which dips. It is clear, therefore, that if we travel from one
hemisphere to the other we must find the northern dip of the needle
gradually diminishing, till at some point near the equator the needle
is horizontal; and as we pass thence to southern regions, a gradually
increasing southern inclination is presented. This has been found to
be the case, and the position of the line along which there is no
inclination (called the _magnetic equator_) has been traced around the
globe. It is not coincident with the earth’s equator, but crosses that
circle at an angle of twelve degrees, passing from north to south of
the equator in long. 3° west of Greenwich, and from south to north in
long. 187° east of Greenwich. The form of the line is not exactly that
of a great circle, but presents here and there (and especially where it
crosses the Atlantic) perceptible excursions from such a figure.
At two points on the earth’s globe the needle will rest in a vertical
position. These are the magnetic poles of the earth. The northern
magnetic pole was reached by Sir J. G. Ross, and lies in 70° N. lat.
and 263° E. long., that is, to the north of the American continent,
and not very far from Boothia Gulf. One of the objects with which
Ross set out on his celebrated expedition to the Antarctic Seas was
the discovery, if possible, of the southern magnetic pole. In this he
was not successful. Twice he was in hopes of attaining his object,
but each time he was stopped by a barrier of land. He approached so
near, however, to the pole, that the needle was inclined at an angle
of nearly ninety degrees to the horizon, and he was able to assign to
the southern pole a position in 75° S. lat., 154° E. long. It is not
probable, we should imagine, that either pole is fixed, since we shall
now see that the inclination, like the declination of the magnetic
needle, is variable from time to time, as well as from place to place;
and in particular, the magnetic equator is apparently subjected to a
slow but uniform process of change.
Arago tells us that the inclination of the needle at Paris has been
observed to diminish year by year since 1671. At that time the
inclination was no less than 75°; in other words, the needle was
inclined only 15° to the vertical. In 1791 the inclination was less
than 71°. In 1831 it was less than 68°. In like manner, the inclination
at London has been observed to diminish, from 72° in 1786 to 70° in
1804, and thence to 68° at the present time.
It might be anticipated from such changes as these that the magnetic
equator would be found to be changing in position. Nay, we can even
guess in which way it must be changing. For since the inclination
is diminishing at London and Paris, the magnetic equator must be
approaching these places, and this (in the present position of the
curve) can only happen by a gradual shifting of the magnetic equator
from east to west along the true equator. This motion has been found to
be really taking place. It is supposed that the movement is accompanied
by a change of form, but more observations are necessary to establish
this interesting point.
Can it be doubted that while these changes are taking place, the
magnetic poles also are slowly shifting round the true pole? Must not
the northern pole, for instance, be further from Paris now that the
needle is inclined more than 23° from the vertical, than in 1671, when
the inclination was only 15°? It appears obvious that this must be so,
and we deduce the interesting conclusion that each of the magnetic
poles is rotating around the earth’s axis.
But there is another peculiarity of the needle which is as noteworthy
as any of those I have mentioned. I refer to the intensity of the
magnetic action—the energy with which the needle seeks its position
of rest. This is not only variable from place to place, but from time
to time, and is further subject to sudden changes of a very singular
character.
It might be expected that where the dip is greater, the directive
energy of the magnet would be proportionately great. And this is found
to be approximately the case. Accordingly, the magnetic equator is
very nearly coincident with the ‘equator of least intensity,’ but not
exactly. As we approach the magnetic poles we find a more considerable
divergence, so that instead of there being a northern pole of greatest
intensity nearly coincident with the northern magnetic pole, which we
have seen lies to the north of the American continent, there are _two_
northern poles, one in Siberia nearly at the point where the river Lena
crosses the Arctic circle, the other not so far to the north—only a
few degrees north, in fact, of Lake Superior. In the south, in like
manner, there are also two poles, one on the Antarctic circle, about
130° E. long., in Adélie Land, the other not yet precisely determined,
but supposed to lie on about the 240th degree of longitude, and south
of the Antarctic circle. Singularly enough, there is a line of lower
intensity running right round the earth along the valleys of the two
great oceans, ‘passing through Behring’s Straits and bisecting the
Pacific, on one side of the globe, and passing out of the Arctic Sea by
Spitsbergen and down the Atlantic, on the other.’
Colonel Sabine discovered that the intensity of the magnetic action
varies during the course of the year. It is greatest in December and
January _in both hemispheres_. If the intensity had been greatest
in winter, one would have been disposed to have assigned seasonal
variation of temperature as the cause of the change. But as the epoch
is the same for both hemispheres, we must seek another cause. Is
there any astronomical element which seems to correspond with the law
discovered by Sabine? There is one very important element. The position
of the perihelion of the earth’s orbit is such that the earth is
nearest to the sun on about the 31st of December or the 1st of January.
There seems nothing rashly speculative, then, in concluding that the
sun exercises a magnetic influence on the earth, varying according
to the distance of the earth from the sun. Nay, Sabine’s results
seem to point very distinctly to the law of variation. For, although
the number of observations is not as yet very great and the extreme
delicacy of the variation renders the determination of its amount very
difficult, enough has been done to show that in all probability the
sun’s influence varies according to the same law as gravity—that is,
inversely as the square of the distance.
That the sun, the source of light and heat, and the great gravitating
centre of the solar system, should exercise a magnetic influence
upon the earth, and that this influence should vary according to the
same law as gravity, or as the distribution of light and heat, will
not appear perhaps very surprising. But the discovery by Sabine that
_the moon_ exercises a distinctly traceable effect upon the magnetic
needle seems to me a very remarkable one. We receive very little light
from the moon, much less (in comparison with the sun’s light) than
most persons would suppose, and we get absolutely no perceptible heat
from her. Therefore it would seem rather to the influence of mass
and proximity that the magnetic disturbances caused by the moon must
be ascribed. But if the moon exercises an influence in this way, why
should not the planets? We shall see that there is evidence of some
such influence being exerted by these bodies.
More mysterious, if possible, than any of the facts I have discussed
is the phenomenon of _magnetic storms_. The needle has been exhibiting
for several weeks the most perfect uniformity of oscillation. Day after
day, the careful microscopic observation of the needle’s progress
has revealed a steady swaying to and fro, such as may be seen in the
masts of a stately ship at anchor on the scarce-heaving breast of
ocean. Suddenly a change is noted; irregular jerking movements are
perceptible, totally distinct from the regular periodic oscillations. A
magnetic storm is in progress. But where is the centre of disturbance,
and what are the limits of the storm? The answer is remarkable. If the
jerking movements observed in places spread over very large regions
of the earth—and in some well-authenticated cases over the whole
earth—be compared with the local time, it is found that (allowance
being made for difference of longitude) _they occur precisely at the
same instant_. The magnetic vibrations thrill in one moment through the
whole frame of our earth!
But a very singular circumstance is observed to characterise these
magnetic storms. They are nearly always observed to be accompanied by
the exhibition of the aurora in high latitudes, northern and southern.
Probably they never happen without such a display, but numbers of
auroras escape our notice. The converse proposition, however, _has_
been established as an universal one. No great display of the aurora
ever occurs without a strongly marked magnetic storm.
Magnetic storms sometimes last for several hours or even days.
Remembering the influence which the sun has been found to exercise upon
the magnetic needle, the question will naturally arise, Has the sun
anything to do with magnetic storms? We have clear evidence that he has.
On the 1st of September, 1859, Messrs. Carrington and Hodgson were
observing the sun, one at Oxford and the other in London. Their
scrutiny was directed to certain large spots which, at that time,
marked the sun’s face. Suddenly a bright light was seen by each
observer to break out on the sun’s surface, and to travel, slowly
in appearance, but in reality at the rate of about 7,000 miles in a
minute, across a part of the solar disc. Now it was found afterwards
that the self-registering magnetic instruments at Kew had made at
that very instant a strongly marked jerk. We learned, also, that at
that moment a magnetic storm prevailed at the West Indies, in South
America, and in Australia. The signalmen in the telegraph stations at
Washington and Philadelphia received strong electric shocks; the pen
of Bain’s telegraph was followed by a flame of fire; and in Norway the
telegraphic machinery was set on fire. At night great auroras were seen
in both hemispheres. It is impossible not to connect these startling
magnetic indications with the remarkable appearance observed upon the
sun’s disc.
But there is other evidence. Magnetic storms prevail more commonly in
some years than in others. In those years in which they occur most
frequently, it is found that the ordinary oscillations of the magnetic
needle are more extensive than usual. Now when these peculiarities had
been noticed for many years, it was found that there was an alternate
and systematic increase and diminution in the intensity of magnetic
action, and that the period of the variation was about eleven years.
But at the same time, a diligent observer had been recording the
appearance of the sun’s face from day to day and from year to year. He
had found that the solar spots are in some years more freely displayed
than in others. And he had determined the period in which the spots are
successively presented with maximum frequency to be about eleven years.
On a comparison of the two sets of observations, it was found (and has
now been placed beyond a doubt by many years of continued observation)
that magnetic perturbations are most energetic when the sun is most
spotted, and _vice versâ_.
For so remarkable a phenomenon as this, none but a cosmical cause can
suffice. We can neither say that the spots cause the magnetic storms
nor that the magnetic storms cause the spots. We must seek for a cause
producing at once both sets of phenomena. There is as yet no certainty
in this matter, but it seems as if philosophers would soon be able to
trace in the disturbing action of the planets upon the solar atmosphere
the cause as well of the marked period of eleven years as of other
less distinctly marked periods which a diligent observation of solar
phenomena is beginning to educe.
(From the _Cornhill Magazine_, June 1868.)
_OUR CHIEF TIME-PIECE LOSING TIME._
A distinguished French astronomer, author of one of the most
fascinating works on popular astronomy that has hitherto appeared,
remarks that a man would be looked upon as a maniac who should speak
of the influence of Jupiter’s moons upon the cotton trade. Yet, as he
proceeds to show, there is an easily traced connection between the
ideas which appear at first sight so incongruous. The link is found in
the determination of celestial longitude.