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Title: Light Science for Leisure Hours - A series of familiar essays on scientific subjects, natural - phenomena, &c.
Author: Proctor, Richard A. (Richard Anthony)
Language: English
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*** Start of this LibraryBlog Digital Book "Light Science for Leisure Hours - A series of familiar essays on scientific subjects, natural - phenomena, &c." ***
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.
Similarly, we should be disposed to wonder at an astronomer who,
regarding thoughtfully the stately motion of the sidereal system,
as exhibited on a magnified, and, therefore, appreciable scale by
a powerful telescope, should speak of the connection between this
movement and the intrinsic worth of a sovereign. The natural thought
with most men would be that ‘too much learning’ had made the astronomer
mad. Yet, when we come to inquire closely into the question of a
sovereign’s intrinsic value, we find ourselves led to the diurnal
motion of the stars, and that by no very intricate path. For, What
is a sovereign? A coin containing so many grains of gold mixed with
so many grains of alloy. A grain, we know, is the weight of such and
such a volume of a certain standard substance—that is, so many cubic
inches, or parts of a cubic inch, of that substance. But what is an
inch? It is determined, we find, as a certain fraction of the length
of a pendulum vibrating seconds in the latitude of London. A second,
we know, is a certain portion of a mean solar day, and is practically
determined by a reference to what is called a sidereal day—the
interval, namely, between the successive passages by the same star of
the celestial meridian of any fixed place. This interval is assumed to
be constant, and it has, indeed, been described as the ‘one constant
element’ known to astronomers.
We find, then, that there is a connection, and a very important
connection, between the motion of the stars and our measures, not
merely of value, but of weight, length, volume, and time. In fact, our
whole system of weights and measures is founded on the apparent diurnal
motion of the sidereal system, that is, on the real diurnal rotation
of the earth. We may look on the meridian-plane in which the great
transit-telescope of the Greenwich Observatory is made to swing, as
the gigantic hand of a mighty dial, a hand which, extending outwards
among the stars, traces out for us, by its motion among them, the exact
progress of time, and so gives us the means of weighing, measuring,
and valuing terrestrial objects with an exactitude which is at present
_beyond_ our wants.
The earth, then, is our ‘chief time-piece,’ and it is of the
correctness of this giant clock that I am now to speak.
But how can we test a time-piece whose motions we select to regulate
every other time-piece? If a man sets his watch every morning by the
clock at Westminster, it is clearly impossible for him to test the
accuracy of that clock by the motions of his watch. It would, indeed,
be possible to detect any gross change of rate; but for the purpose of
illustration I assume, what is indeed the case, that the clock is very
accurate, and therefore that minute errors only are to be looked for
even in long intervals of time. And just as the watch set by a clock
cannot be made use of to test the clock for small errors, so our best
time-pieces cannot be employed to detect slow variations, if any such
exist, in the earth’s rotation-period.
Sir William Herschel, who early saw the importance of the subject,
suggested another method. Some of the planets rotate in such a manner,
and bear such distinct marks upon their surface, that it is possible,
by a series of observations extending over a long interval of time, to
determine the length of their rotation-period within a second or two.
Supposing their rotation uniform, we at once obtain an accurate measure
of time. Supposing their rotation _not_ uniform, we obtain—(1) a hint
of the kind of change we are looking for; and (2), by the comparison of
two or more planets, the means of guessing how the variation is to be
distributed between the observed planets and our earth.
Unfortunately, it turned out that Jupiter, one of the planets
from which Herschel expected most, does _not_ afford us exact
information-his real surface being always veiled by his dense and
vapour-laden atmosphere. Saturn, Venus, and Mercury are similarly
circumstanced, and are in other respects unfavourable objects for
this sort of observation. Mars only, of all the planets, is really
available. Distinctly marked (in telescopes of sufficient power) with
continents and oceans, which are rarely concealed by vapours, this
planet is in other respects fortunately situated. For it is certain
that whatever variations may be taking place in planetary rotations
must be due to external agencies. Now, Saturn and Jupiter have their
satellites to influence (perhaps appreciably in long intervals of time)
their rotation-movements. Venus and Mercury are near the sun, and are
therefore in this respect worse off than the earth, whose rotation
is in question. Mars, on the other hand, farther removed than we are
from the sun, having also no moon, and being of small dimensions
(a very important point, be it observed, since the tidal action of
the sun depends on the dimensions of a planet), is likely to have a
rotation-period all but absolutely constant.
Herschel was rather unfortunate in his observations of Mars. Having
obtained a rough approximation from Mars’ rotation in an interval
of two days—this rough approximation being, as it chanced, only
thirty-seven seconds in excess of the true period, he proceeded to
take three intervals of one month each. This should have given a much
better value; but, as it happened, the mean of the values he obtained
was forty-six seconds too great. He then took a period of two years,
and being misled by the erroneous values he had already obtained, he
_missed one rotation_, getting a value two minutes too great. Thirty
years ago, two German astronomers, Beer and Madler, tried the same
problem, and taking a period of seven years, obtained a value which
exceeds the true value by only one second. Another German, Kaiser,
by combining more observations, obtained a value which is within
one-fifteenth of a second of the true value. But a comparison of
observations extending over 200 years has enabled me to obtain a value
which I consider to lie within one-hundredth part of a second of the
truth. This value for Mars’ rotation-period is 24 hours 37 minutes
22·73 seconds.
Here, then, we have a result so accurate, that _at some future time_
it may serve to test the earth’s rotation-period. We have compared
the rotation-rate of our test-planet with the earth’s rate during the
past 200 years; and therefore, if the earth’s rate vary by more than
one-hundredth of a second in the next two or three hundred years, we
shall—or rather our descendants will—begin to have some notion of the
change at the end of that time.
But in the meantime, mankind being impatient, and not willing to leave
to a distant posterity any question which can possibly be answered
_now_, astronomers have looked around them for information available at
once on this interesting point. The search has not been in vain. In
fact, we are able to announce, with an approach to positiveness, that
our great terrestrial time-piece is actually _losing time_.
In our moon we have a neighbour which has long been in the habit of
answering truthfully questions addressed to her by astronomers. Of
old, she told Newton about gravitation, and when he doubted, and
urged opposing evidence offered—as men in his time supposed—by the
earth, she set him on the right track, so that when in due time the
evidence offered by the earth was corrected, Newton was prepared
at once to accept and propound the noble theory which rendered his
name illustrious. Again, men wished to learn the true shape of the
earth, and went hither and thither measuring its globe; but the moon,
meanwhile, told the astronomer who remained at home a truer tale. They
sought to learn the earth’s distance from the sun, and from this and
that point they turned their telescopes on Venus in transit; but the
moon set them nearer the truth, and that not by a few miles, but by
2,000,000 miles or more. We shall see that she has had something to say
about our great terrestrial time-piece.
One of the great charms of the science of astronomy is, that it enables
men to _predict_. At such and such an hour, the astronomer is able
to say, a celestial body will occupy such and such a point on the
celestial sphere. You direct a telescope towards the point named, and
lo! at the given instant, the promised orb sweeps across the field
of view. Each year there is issued a thick octavo volume crowded
with such predictions, three or four years in advance of the events
predicted; and these predictions are accepted with as little doubt by
astronomers as if they were the records of past events.
But astronomers are not only able to predict—they can also trace
back the paths of the celestial bodies, and say: ‘At such and such
a long-past epoch, a given star or planet occupied such and such a
position upon the celestial sphere.’ But how are they to verify such
a statement? It is clear that, in general, they cannot do so. Those
who are able to appreciate (or better, to make use of) the predictions
of astronomy, will, indeed, very readily accord a full measure of
confidence to calculations of past events. They know that astronomy
is justly named the most exact of the sciences, and they can see that
there is nothing, in the nature of things, to render retrospection
more difficult than prevision. But there are hundreds who have no such
experience of the exactness of modern astronomical methods—who have,
on the contrary, a vague notion that modern astronomy is merely the
successor of systems now exploded; perhaps even that it may one day
have to make way in its turn for new methods. And if all other men were
willing to accept the calculations of astronomers respecting long-past
events, astronomers themselves would be less easily satisfied. Long
experience has taught them that the detection of error is the most
fruitful source of knowledge; therefore, wherever such a course is
possible, they always gladly submit their calculations to the test of
observation.
Now, looking backward into the far past, it is only here and there
that we see records which afford means of comparison with modern
calculations. The planets had swept on in their courses for ages
with none to note them. Gradually, observant men began to notice and
record the more remarkable phenomena. But such records, made with very
insufficient instrumental means, had in general but little actual
value: it has been found easy to confirm them without any special
regard to accuracy of calculation.
There is one class of phenomena, however, which no inaccuracy of
observation can very greatly affect. A total eclipse of the sun is an
occurrence so remarkable, that (1) it can hardly take place without
being recorded, and (2) a very rough record will suffice to determine
the particular eclipse referred to. Long intervals elapse between
successive total eclipses visible at the same place on the earth’s
surface, and even partial eclipses of noteworthy extent occur but
seldom at any assigned place. Very early, therefore, in the history of
modern astronomy, the suggestion was made, that eclipses recorded by
ancient historians should be calculated retrospectively. An unexpected
result rewarded the undertaking. It was found that ancient eclipses
could not be fairly accounted for without assigning a slower motion to
the moon in long-past ages than she has at present!
Here was a difficulty which long puzzled mathematicians. One after
another was foiled by it. Halley, an English mathematician, had
detected the difficulty, but no English mathematician was able to
grapple with it. Contented with Newton’s fame, they had suffered their
Continental rivals to shoot far ahead in the course he had pointed out.
But the best Continental mathematicians were defeated. In papers of
acknowledged merit, adorned by a variety of new processes, and showing
a deep insight into the question at issue, they yet arrived, one and
all, at the same conclusion—failure.
Ninety years elapsed before the true explanation was offered by the
great mathematician Laplace. A full exposition of his views would be
out of place in such a paper as the present, but, briefly, they amount
to this:—
The moon travels in her orbit, swayed chiefly by the earth’s
attraction. But the sun, though greatly more distant, yet, owing to
the immensity of his mass, plays an important part in guiding our
satellite. His influence tends to relieve the moon, in part, from the
earth’s sway. Thus she travels in a wider orbit, and with a slower
motion, than she would have but for the sun’s influence. Now the earth
is not at all times equally distant from the sun, and his influence
upon the moon is accordingly variable. In winter, when the earth
is nearest to the sun, his influence is greatest. The lunar month,
accordingly (though the difference is very slight), is longer in
winter than in summer. This variation had long been recognised as the
moon’s ‘annual equation;’ but Laplace was the first to point out that
the variation is itself slowly varying. The earth’s orbit is slowly
changing in shape—becoming more and more nearly circular year by year.
As the greater axis of her orbit is unchanging, it is clear that the
actual extent of the orbit is slowly increasing. Thus, the moon is
slightly released from the sun’s influence year by year, and so brought
more and more under the earth’s influence. She travels, therefore,
continually faster and faster, though the change is indeed but a very
minute one;—only to be detected in long intervals of time. Also the
moon’s _acceleration_, as the change is termed, is only temporary, and
will in due time be replaced by an equally gradual retardation.
When Laplace had calculated the extent of the change due to the cause
he had detected, and when it was found that ancient eclipses were
now satisfactorily accounted for, it may well be believed that there
was triumph in the mathematical camp. But this was not all. Other
mathematicians attacked the same problem, and their results agreed so
closely that all were convinced that the difficulty was thoroughly
vanquished.
A very noteworthy result followed from Laplace’s calculations.
Amongst other solutions which had been suggested, was the supposition
(supported by no less an authority than Sir Isaac Newton, who lived to
see the commencement of the long conflict maintained by mathematicians
with this difficulty), that it is not the moon travelling more quickly,
but our earth rotating more slowly, which causes the observed
discrepancy. Now it resulted from Laplace’s labours—as he was the first
to announce—that the period of the earth’s rotation has not varied by
one-tenth of a second per century in the last two thousand years.
The question thus satisfactorily settled, as was supposed, was
shelved for more than a quarter of a century. The result, also, which
seemed to flow from the discussion—the constancy of the earth’s
rotation-movement—was accepted; and, as we have seen, our national
system of measures was founded upon the assumed constancy of the day’s
duration.
But mathematicians were premature in their rejoicings. The question
has been brought, by the labours of Professor Adams—co-discoverer
with Leverrier of the distant Neptune—almost exactly to the point
which it occupied a century ago. We are face to face with the very
difficulties—somewhat modified in extent, but not in character—which
puzzled Halley, Euler, and Lagrange. It would be an injustice to
the memory of Laplace to say that his labours were thrown away.
The explanation offered by him is indeed a just one. But it is
insufficient. Properly estimated it removes only half the difficulty
which had perplexed mathematicians. It would be quite impossible to
present in brief space, and in form suited to these pages, the views
propounded by Adams. What, for instance, would most of our readers
learn if we were to tell them that, ‘when the variability of the
eccentricity is taken into account, in integrating the differential
equations involved in the problem of the lunar motions—that is, when
the eccentricity is made a function of the time—non-periodic or secular
terms appear in the expression for the moon’s mean motion’—and so
on? Let it suffice to say that Laplace had considered only the work
of the sun in diminishing the earth’s _pull_ on the moon, supposing
that the slow variation in the sun’s _direct_ influence on the moon’s
motion in her orbit must be self-compensatory in long intervals of
time. Adams has shown, on the contrary, that when this variation is
closely examined, no such compensation is found to take place; and that
the effect of this want of compensation is to diminish by more than
one-half the effects due to the slow variation examined by Laplace.
These views gave rise at first to considerable controversy.
Pontécoulant characterised Adams’s processes as ‘analytical
conjuring-tricks,’ and Leverrier stood up gallantly in defence of
Laplace. The contest swayed hither and thither for a while, but
gradually the press of new arrivals on Adams’s side began to prevail.
One by one his antagonists gave way; new processes have confirmed his
results, figure for figure; and no doubt now exists, in the mind of any
astronomer competent to judge, of the correctness of Adams’s views.
But, side by side with this inquiry, another had been in progress. A
crowd of diligent labourers had been searching with close and rigid
scrutiny into the circumstances attending ancient eclipses. A new light
had been thrown upon this subject by the labours of modern travellers
and historians. One remarkable instance of this may be cited. Mr.
Layard has identified the site of Larissa with the modern Nimroud.
Now, Xenophon relates that when Larissa was besieged by the Persians,
an eclipse of the sun took place, so remarkable in its effects (and
therefore undoubtedly total), that the Median defenders of the town
threw down their arms, and the city was accordingly captured. And
Hansen has shown that a certain estimate of the moon’s motion makes the
eclipse which occurred on August 15, 310 B.C., not only _total_, but
_central_ at Nimroud. Some other remarkable eclipses—as the celebrated
sunset eclipse (total) at Rome, 399 B.C.; the eclipse which enveloped
the fleet of Agathocles as he escaped from Syracuse; the famous eclipse
of Thales, which interrupted a battle between the Medes and Lydians;
and even the partial eclipse which (possibly) caused the ‘going back
of the shadow upon the dial of Ahaz’—have all been accounted for
satisfactorily by Hansen’s estimate of the moon’s motion: so also have
nineteen lunar eclipses recorded in the Almagest.
This estimate of Hansen’s, which accounts so satisfactorily for solar
and lunar eclipses, makes the moon’s rate of motion increase more than
twice as fast as it should do according to the calculations of Adams.
But before our readers run away with the notion that astronomers have
here gone quite astray, it will be well to present, in a simple manner,
the extreme minuteness of the discrepancy about which all the coil has
been made.
Suppose that, just in front of our moon, a false moon exactly equal to
ours in size and appearance (see _note_ at the end of this paper) were
to set off with a motion corresponding to the present motion of the
moon, save only in one respect—namely, that the false moon’s motion
should not be subject to the change we are considering, termed _the
acceleration_. Then one hundred years would elapse before our moon
would fairly begin to show in advance. She would, in that time, have
brought only one one-hundred-and-fiftieth part of her breadth from
behind the false moon. At the end of another century she would have
gained four times as much; at the end of a third, nine times as much:
and so on. She would not fairly have cleared her own breadth in less
than twelve hundred years. But the _whole_ of this gain, minute as it
is, is not left unaccounted for by our modern astronomical theories.
_Half_ the gain is explained, the other half remains to be interpreted;
in other words, _the moon travels further by about half her own breadth
in twelve centuries than she should do according to the lunar theory_.
But in this difficulty, small as it seems, we are not left wholly
without resource. We are not only able to say that the discrepancy is
probably due to a gradual retardation of the earth’s rotation-movement,
but we are able to place our finger on a very sufficient cause for such
a retardation. One of the most firmly established principles of modern
science is this—that where _work is done_, force is, in some way or
other, expended. The _doing of work_ may show itself in a variety of
ways— in the generation of heat, in the production of light, in the
raising of weights, and so on; but in every case an equivalent force
must be expended. If the brakes are applied to a train in motion,
intense heat is generated in the substance of the brake. Now, the force
employed by the brakesman is _not_ equivalent to the heat generated.
Where, then, is the balance of force expended? We all know that the
train’s motion is retarded, and this loss of motion represents the
requisite expenditure of force. Now, is there any process in nature
resembling, in however remote a degree, the application of a brake to
check the earth’s rotation? There is. The tidal wave, which sweeps,
twice a day, round the earth, travels in a direction contrary to the
earth’s motion of rotation. That this wave ‘does work,’ no one can
doubt who has watched its effects. The mere rise and fall in open
ocean may not be strikingly indicative of ‘work done;’ but when we
see the behaviour of the tidal wave in narrow channels, when we see
heavily-laden ships swept steadily up our tidal rivers, we cannot but
recognise the expenditure of force. Now, where does this force come
from? Motion being the great ‘force-measurer,’ what motion _suffers_
that the tides may _work_? We may securely reply, that the only
motion which _can_ supply the requisite force is the earth’s motion
of rotation. Therefore, it is no mere fancy, but a matter of absolute
certainty, that, though slowly, still very surely, our terrestrial
globe is losing its rotation-movement.
Considered as a time-piece, what are the earth’s errors? Suppose, for
a moment, that the earth was _timed_ and _rated_ two thousand years
ago, how much has she _lost_, and what is her ‘rate-error?’ She has
lost in that interval nearly one hour and a quarter, and she is losing
now at the rate of one second in twelve weeks. In other words, the
length of a day is now more by about one eighty-fourth part of a second
than it was two thousand years ago. At this rate of change, our day
would merge into a lunar month in the course of thirty-six thousand
millions of years. But after a while, the change will take place more
slowly, and some trillion or so of years will elapse before the full
change is effected.
Distant, however, as is the epoch at which the changes we have been
considering will become effective, the subject appears to us to have
an interest apart from the mere speculative consideration of the
future physical condition of our globe. Instead of the recurrence of
ever-varying, closely intermingled cycles of fluctuation, we see, now
for the first time, the evidence of cosmical decay—a decay which, in
its slow progress, may be but the preparation for renewed genesis—but
still, a decay which, so far as the races at present subsisting upon
the earth are concerned, must be looked upon as finally and completely
destructive.[2]
(From _Chambers’s Journal_, October 12, 1867.)
_ENCKE THE ASTRONOMER._
The years which have passed since Encke died have witnessed notable
changes in the aspect of the science he loved so well. But we must look
back over more than half a century, if we would form an estimate of
the position of astronomy when Encke’s most notable work was achieved.
At Seeberge, under Lindenau, Encke had been perfecting himself in the
higher branches of mathematical calculation. He took the difficult
work of determining the orbital motions of newly-discovered comets
under his special charge, and Dr. Bruhns tells us that every comet
which was detected during Encke’s stay at Seeberge was subjected to
rigid scrutiny by the indefatigable mathematician. Before long a
discovery of the utmost importance rewarded his persevering labours.
Pons had detected on November 26, 1818, a comet of no very brilliant
aspect, which was watched first at Marseilles, and then at Mannheim,
until December 29. Encke next took up the work, and tracked the comet
until January 12. Combining the observations made between December 22
and January 12, he assigned to the body a parabolic orbit. But he was
not satisfied with the accordance between this path and the observed
motions of the body. When he attempted to account for the motions of
the comet by means of an orbit of comparatively short period, he was
struck by the resemblance between the path thus deduced and that of
Comet I, 1805. Gradually the idea dawned upon him that a new era was
opening for science. Hitherto the only periodical comets which had been
discovered except Lexell’s—the ‘lost comet’—had travelled in orbits
extending far out into space beyond the paths of the most distant known
planets. But now Encke saw reason to believe that he had to deal with a
comet travelling within the orbit of Jupiter. On February 5, he wrote
to the eminent mathematician Gauss, pointing out the results of his
inquiries, and saying that he only waited for the encouragement and
authority of his former teacher to prosecute his researches to the end
towards which they already seemed to point. Gauss, in reply, not only
encouraged Encke to proceed, but counselled him as to the course he
should pursue. The result we all know. Encke showed conclusively that
the newly-discovered comet travels in a path of short period, and that
it had already made its appearance several times in our neighbourhood.
From the date of this discovery, Encke took high rank among the
astronomers of Europe. His subsequent labours by no means fell
short of the promise which this, his first notable achievement, had
afforded. If he effected less as an astronomical observer than many
of his contemporaries, he was surpassed by few as a manipulator of
those abstruse formulæ by which the planetary perturbations are
calculated. It was to the confidence engendered by this skill that
we owe his celebrated discovery of the acceleration of the motion of
the comet mentioned above. Assured that he had rightly estimated the
disturbances to which the comet is subjected, he was able to pronounce
confidently that some cause continually (though all but imperceptibly)
impedes the passage of this body through space, and so—by one of
those strange relations which the student of astronomy is familiar
with—the continually retarded comet travels ever more swiftly along a
continually diminishing orbit.
Bruhns’ Life of Encke is well worth reading, not only by those who
are interested in Encke’s fame and work as an astronomer, but by
the general reader. Encke the man is presented to our view, as well
as Encke the astronomer. With loving pains the pupil of the great
astronomer handles the theme he has selected. The boyhood of Encke, his
studies, his soldier life in the great uprising against Napoleon in
1813, and his work at the Seeberge Observatory; his labours on comets
and asteroids; his investigations of the transits of 1761 and 1769; his
life as an academician, and as director of an important observatory;
his orations at festival and funeral; and lastly, his illness and
death, are described in these pages by one who held Encke in grateful
remembrance as ‘teacher and master,’ and as a ‘fatherly friend.’
Not the least interesting feature of the work is the correspondence
introduced into its pages. We find Encke in communication with
Humboldt, with Bessel and Struve, with Hansen, Olbers, and Argelander;
with a host, in fine, of living as well as of departed men of science.
(From _Nature_, March 10, 1870.)
FOOTNOTES:
[1] Other green lines have since been discovered in the auroral
spectrum; and occasionally a red line is seen.
[2] In the _Quarterly Journal of Science_ for October 1866, a more
detailed but somewhat less popular account of the subject of the above
paper is presented. A few months earlier, a skilfully-written paper
on the same subject, from the pen of Mr. J. M. Wilson, of Rugby, had
appeared in the _Eagle_, a magazine written by and for members of
St. John’s College, Cambridge. Although my paper in the _Quarterly
Journal of Science_ was written quite independently of Mr. Wilson’s
(which, however, I had read), yet it chanced that in describing the
same mathematical relations, and the same sequence of events, I here
and there used language closely resembling his. I fear this led for a
while to some misconception; but I was fortunately able to show in Mr.
De la Rue’s address to the Astronomical Society, on the same subject,
passages yet more strikingly resembling some in Mr. Wilson’s paper
(written subsequently and quite independently). The fact would seem to
be that if two persons describe exactly the same events, and deal with
exactly the same mathematical relations, it is almost certain that in
more than one passage they will use somewhat similar expressions.
I was actually indebted to Mr. Wilson’s paper for one illustration,
however,—that derived from the movements of a supposed artificial
moon; and I think that had his paper appeared in a magazine printed
for general circulation, I should have referred to it. As it was,
this seemed useless so far as the readers of the _Quarterly Journal
of Science_ were concerned. The circumstances of the case were,
indeed, far from calling for a reference; while I had in a sense made
the illustration my own by detecting an important miscalculation in
the original (the amount of advance being either doubled or halved—I
forget which). Had I referred to Mr. Wilson’s paper, I must needs have
mentioned this mistake; and it would have appeared as though I had had
no other purpose in making the reference.
I mention these matters to explain what I fear my esteemed
fellow-collegian was disposed at the time to regard as either a wrong
or a slight. Nothing was further from my intention than either.
_VENUS ON THE SUN’S FACE._
More than a century ago scientific men were looking forward with eager
interest to the passage of the planet Venus across the sun’s face in
1769. The Royal Society judged the approaching event to be of such
extreme importance to the science of astronomy that they presented a
memorial to King George III., requesting that a vessel might be fitted
out, at Government expense, to convey skilful observers to one of the
stations which had been judged suitable for observing the phenomenon.
The petition was complied with, and after some difficulty as to the
choice of a leader, the good ship ‘Endeavour,’ of 370 tons, was placed
under the command of Captain Cook. The astronomical work entrusted to
the expedition was completely successful; and thus it was held that
England had satisfactorily discharged her part of the work of utilising
the rare phenomenon known as a transit of Venus.
A century passed, and science was again awaiting with interest the
approach of one of these transits. But now her demands were enlarged.
It was not one ship that was asked for, but the full cost and charge of
several expeditions. And this time, also, science had been more careful
in taking time by the forelock. The first hints of her requirements
were heard some fourteen years ago, when the Astronomer-Royal began
that process of laborious inquiry which a question of this sort
necessarily demands. Gradually, her hints became more and more
plain-spoken; insomuch that Airy—her mouthpiece in this case—stated
definitely in 1868 what he thought science had a right to claim from
England in this matter. When the claim came before our Government, it
was met with a liberality which was a pleasing surprise after some
former placid references of scientific people to their own devices.
The sum of ten thousand five hundred pounds was granted to meet the
cost of several important and well-appointed expeditions; and further
material aid was derived from the various Government observatories.
And now let us inquire why so much interest is attached to a phenomenon
which appears, at first sight, to be so insignificant. Transits,
eclipses, and other phenomena of that nature are continually occurring,
without any particular interest being attached to them. The telescopist
may see half-a-dozen such phenomena in the course of a night or two,
by simply watching the satellites of Jupiter, or the passage of our
moon over the stars. Even the great eclipse of 1868 did not attract so
much interest as the transit of Venus; yet that eclipse had not been
equalled in importance by any which has occurred in historic times,
and hundreds of years must pass before such another happens, whereas
transits of Venus are far from being so uncommon.
The fact is, that Venus gives us the best means we have of mastering a
problem which is one of the most important within the whole range of
the science of astronomy. I use the term important, of course, with
reference to the scientific significance and interest of the problem.
Practically, it matters little to us whether the sun is a million of
miles or a thousand millions of miles from us. The subject must in
any case be looked upon as an extra-parochial one. But science does
occasionally attach immense interest to extra-parochial subjects. And
this is neither unwise nor unreasonable, since we find implanted
in our very nature—and not merely in the nature of scientific men—a
quality which causes us to take interest in a variety of matters
that do not in the least concern our personal interests. Nor is this
quality, rightly considered, one of the least noble characteristics of
the human race.
That the determination of the sun’s distance is important, in an
astronomical sense, will be seen at once when it is remembered that
the ideas we form of the dimensions of the solar system are wholly
dependent on our estimate of the sun’s distance. Nor can we gauge the
celestial depths with any feeling of assurance, unless we know the
true length of that which is our sole measuring-rod. It is, in fact,
our basis of measurement for the whole visible universe. In some
respects, even if we knew the sun’s distance exactly, it would still
be an unsatisfactory gauge for the stellar depths. But that is the
misfortune, not the fault, of the astronomer, who must be content to
use the measuring-rod which nature gives him. All he can do is to find
out as nearly as possible its true length.
When we come to consider how the astronomer is to determine this
very element—the sun’s distance—we find that he is hampered with a
difficulty of precisely the same character.
The sun being an inaccessible object, the astronomer can apply no other
methods to determine its distance—directly—than those which a surveyor
would use in determining the distance of an inaccessible castle, or
rock, or tree, or the like. We shall see presently that the ingenuity
of astronomers has, in fact, suggested some other indirect methods.
But clearly the most satisfactory estimate we can have of the sun’s
distance is one founded on such simple notions and involving in the
main such processes of calculation as we have to deal with in ordinary
surveying.
There is, in this respect, no mystery about the solution of the famous
problem. Unfortunately, there is enormous difficulty.
When a surveyor has to determine the distance of an inaccessible
object, he proceeds in the following manner. He first very carefully
measures a base-line of convenient length. Then from either end of
the base-line he takes the bearing of the inaccessible object—that
is, he observes the direction in which it lies. It is clear that, if
he were now to draw a figure on paper, laying down the base-line to
some convenient scale, and drawing lines from its ends in directions
corresponding to the bearings of the observed object, these lines would
indicate, by their intersection, the true relative position of the
object. In practice, the mathematician does not trust to so rough a
method as construction, but applies processes of calculation.
Now, it is clear that in this plan everything depends on the base-line.
It must not be too short in comparison with the distance of the
inaccessible object; for then, if we make the least error in observing
the bearings of the object, we get an important error in the resulting
determination of the distances. The reader can easily convince himself
of this by drawing an illustrative case or two on paper.
The astronomer has to take his base-line for determining the sun’s
distance, upon our earth, which is quite a tiny speck in comparison
with the vast distance which separates us from the sun. It had been
found difficult enough to determine the moon’s distance with such a
short base-line to work from. But the moon is only about a quarter of
a million of miles from us, while the sun is more than ninety millions
of miles off. Thus the problem was made several hundred times more
difficult—or, to speak more correctly, it was rendered simply insoluble
unless the astronomer could devise some mode of observing which should
vastly enhance the power of his instruments.
For let us consider an illustrative case. Suppose there was a steeple
five miles off, and we had a base-line only two feet long. That would
correspond as nearly as possible to the case the astronomer has to deal
with. Now, what change of direction could be observed in the steeple
by merely shifting the eye along a line of two feet? There is a ready
way of answering. Invert the matter. Consider what a line of two feet
long would look like if viewed from a distance of five miles. Would
its length be appreciable, to say nothing of its being measurable? Yet
it is just such a problem as the measurement of that line which the
astronomer would have to solve.
But even this is not all. In our illustration only one observer is
concerned, and he would be able to use one set of instruments.
Suppose, however, that from one end of the two-feet line an observer
using one set of instruments took the bearings of the steeple; and
that, half a year after, another observer brought another set of
instruments and took the bearing of the steeple from the other end of
the two-feet line, is it not obvious how enormously the uncertainty
of the result would be increased by such an arrangement as this? One
observer would have his own peculiar powers of observation, his own
peculiar weaknesses: the other would have different peculiarities. One
set of instruments would be characterised by its own faults or merits,
so would the other. One series of observations would be made in summer,
with all the disturbing effects due to heat; the other would be made in
winter, with all the disturbing effects due to cold.
The observation of the sun is characterised by all these difficulties.
Limited to the base-lines he can measure on earth, the astronomer must
set one observer in one hemisphere, another in the other. Each observer
must have his own set of instruments; and every observation which one
has made in summer will have to be compared with an observation which
the other has made in winter.
Thus we can understand that astronomers should have failed totally when
they attempted to determine the sun’s distance without aid from the
other celestial bodies.
It may seem at first sight as though nothing the other celestial bodies
could tell the astronomer would be of the least use to him, since
these bodies are for the most part farther off than the sun, and even
those which, approach nearest to us are still far beyond the limits of
distance within which the simple plan followed by surveyors could be of
any service. And besides, it might be supposed that information about
the distance of one celestial body could be of no particular service
towards the determination of the distance of another.
But two things aid the astronomer at this point. First of all, he has
discovered the law which associates together the distances of all the
planets from the sun; so that if he can determine the distance of any
one planet, he learns immediately the distances of all. Secondly, the
planets in their motion travel occasionally into such positions that
they become mighty indices, tracing out on a natural dial-plate the
significant lesson from which the astronomer hopes to learn so much.
To take an instance from the motions of another planet than the one
we are dealing with. Mars comes sometimes so near the earth that the
distance separating us from him is little more than one-third of that
which separates us from the sun. Suppose that, at such a time, he
is seen quite close to a fixed star. That star gives the astronomer
powerful aid in determining the planet’s distance. For, to observers in
some parts of the earth, the planet will seem nearer to the star than
he will to observers elsewhere. A careful comparison of the effects
thus exhibited will give significant evidence respecting the distance
of Mars. And we see that the star has served as a fixed mark upon the
vast natural dial of the heavens, just as the division-marks on a
clock-face serve to indicate the position of the hands.
Now we can at once see why Venus holds so important a position in this
sort of inquiry. Venus is our nearest neighbour among the planets.
She comes several millions of miles nearer to us than Mars, our next
neighbour on the other side. That is the primary reason of her being
so much considered by astronomers. But there is another of equal
importance. Venus travels nearer than our earth to the sun. And thus
there are occasions when she gets directly between the earth and the
sun. At those times she is seen upon his face, and his face serves as
a dial-plate by which to measure her movements. When an observer at
one part of the earth sees her on one part of the sun’s face, another
observer at some other part of the earth will see her on another, and
the difference of position, if accurately measured, would at once
indicate the sun’s distance. As a matter of fact, other modes of
reading off the indications of the great dial-plate have to be adopted.
Before proceeding to consider those modes, however, we must deal with
one or two facts about Venus’s movements which largely affect the
question at issue.
Let us first see what we gain by considering the distance of Venus
rather than that of the sun.
At the time of a transit Venus is of course on a line between the earth
and the sun, and she is at somewhat less than a third of the sun’s
distance from us. Thus whatever effect an observer’s change of place
would produce upon the sun would be more than trebled in the case of
Venus. But it must not be forgotten that we are to judge the motions
of Venus by means of the dial-plate formed by the solar disc, and that
dial-plate is itself shifted as the observer shifts his place. Venus is
shifted three times as much, it is true; but it is only the balance of
change that our astronomer can recognise. That balance is, of course,
rather more than twice as great as the sun’s change of place.
So far, then, we have not gained much, since it has been already
mentioned that the sun’s change of place is not measurable by any
process of observation astronomers can apply.
It is to the fact that we have the sun’s disc, whereby to measure the
change, that we chiefly trust; and even that would be insufficient were
it not for the fact that Venus is not at rest, but travels athwart the
great solar dial-plate. We are thus enabled to make a time measurement
take the place of a measurement of space. If an observer in one place
sees Venus cross the sun’s face at a certain distance from the centre,
while an observer at another place sees her follow a path slightly
farther from the centre, the transit clearly seems longer to the former
observer than to the latter.
This artifice of exchanging a measurement of time for one of space—or
_vice versâ_—is a very common one among astronomers. It was Edmund
Halley, the friend and pupil of Sir Isaac Newton, who suggested its
application in the way above described. It will be noticed that what
is required for the successful application of the method is that one
set of observers should be as far to the north as possible, another as
far to the south, so that the path of Venus may be shifted as much as
possible. Clearly the northern observers will see her path shifted as
much to the south as it can possibly be, while the southern observers
will see the path shifted as far as possible towards the north.
One thing, however, is to be remembered. A transit lasts several hours,
and our observers must be so placed that the sun will not set during
these hours. This consideration sometimes involves a difficulty. For
our earth does not supply observing room all over her surface, and the
region where observation would be most serviceable may be covered by
a widely-extended ocean. Then again, the observing parties are being
rapidly swayed round by the rotating earth and it is often difficult
to fix on a spot which may not, through this cause, be shifted from a
favourable position at the beginning of the transit to an unfavourable
one at the end.
Without entering on all the points of difficulty involved by such
considerations as these, I may simply indicate the fact that the
astronomer has a problem of considerable complexity to solve in
applying Halley’s method of observation to a transit of Venus.
It was long since pointed out by the French astronomer Delisle that the
subject may be attacked another way—that, in fact, instead of noticing
how much longer the transit lasts in some places than in others, the
astronomer may inquire how much earlier it begins or ends in some
places than in others.
Here is another artifice, extremely simple in principle, though not
altogether so simple in its application. My readers must bear with
me while I briefly describe the qualities of this second method,
because in reality the whole question of the transit, and all the
points which have to be attended to in the equipment and placing of
the various observing parties, depend on these preliminary matters.
Without attending to them—or at least to such primary points as I
shall select—it would be impossible to form a clear conception of the
circumstances with which astronomers have to deal. There is, however,
no real difficulty about this part of the subject, and I shall only ask
of the reader to give his attention to it for a very brief space of
time.
Suppose the whole of that hemisphere of the earth on which the sun is
shining when the transit is about to begin were covered with observers
waiting for the event. As Venus sweeps rapidly onwards to the critical
part of her path, it is clear that some of these observers will get
an earlier view of the commencement of the transit than others will;
just as at a boat-race, persons variously placed round a projecting
corner of the course see the leading boat come into view at different
times. Some one observer on the outer rim of the hemisphere would be
absolutely the first to see the transit begin. Then rapidly other
observers would see the phenomenon; and in the course of a few minutes
some one observer on the outer rim of the hemisphere—almost exactly
opposite the first—would be absolutely the last to see the transit
begin. From that time the transit would be seen by all for several
hours—I neglect the earth’s rotation, for the moment—but the end of the
transit, like the beginning, would not be seen simultaneously by the
observers. First one would see it, then in succession the rest, and
last of all an observer almost exactly opposite the first.
Now, here we have had to consider four observers who occupy exceptional
positions. There is (1) the observer who sees the transit begin
earliest, (2) the one who sees it begin latest, (3) the one who sees it
end earliest, and (4) the one who sees it end latest. Let us consider
the first two only. Suppose these two observers afterwards compared
notes, and found out what was the exact difference of time between
their respective observations. Is it not clear that the result would
at once afford the means of determining the sun’s distance? It would
be the simplest of all possible astronomical problems to determine
over what proportion of her orbit Venus passed in the interval of time
which elapsed between these observations; and the observers would now
have learned that that portion of Venus’s orbit is so many miles long,
for they know what distance separated them, and it would be easy to
calculate how much less that portion of Venus’s orbit is. Thus they
would learn what the length of her whole orbit is, thence her distance
from the sun, and thence the sun’s distance from us.
The two observers who saw the transit end earliest and latest could do
the like.
Speaking generally, and neglecting all the complexities which delight
the soul of the astronomer, this is Delisle’s method of utilising a
transit. It has obviously one serious disadvantage as compared with
the other. An observer at one side of the earth has to bring his
observations into comparison with those made by an observer at the
other side of the earth. Each uses the local time of the place at which
he observes, and it has been calculated that for the result to be of
value there must not be an error of a single second in their estimates
of local time. Now, does the reader appreciate the full force of this
proviso? Each observer must know so certainly in what exact longitude
he is, that his estimate of the time when true noon occurs shall not be
one second wrong! This is all satisfactory enough in places where there
are regular observatories. But matters are changed when we are dealing
with such places as Woahoo, Kerguelen Land, Chatham Island, and the
wilds of Siberia.
In the transit[3] of 1874 there are many such difficulties to be
encountered. In fact, it is almost impossible to conceive a transit
the circumstances of which are more inconvenient. On the other hand,
however, the transit is of such a nature that if once the preliminary
difficulties are overcome, we can hope more from its indications than
from those of any other transit which will happen in the course of the
next few centuries.
The transit will begin earliest for observers in the neighbourhood
of the Sandwich Islands, latest for observers near Crozet Island,
far to the south-east of the Cape of Good Hope. It ends earliest for
observers far to the south-west of Cape Horn, latest for observers in
the north-eastern part of European Russia. Thus we see that, so far as
the application of our second method is concerned, the suitable spots
are not situated in the most inviting regions of the earth’s surface.
As the transit happens on December 8, 1874, the principal northern
stations will be very bleak abodes for the observers. The southern
stations are in yet more dreary regions,—notwithstanding the fact that
the transit occurs during the summer of the southern hemisphere.
For the application of Halley’s method we require stations where
the whole transit will be visible; and as the days are very short
at the northern stations in December, it is as respects these that
we encounter most difficulty. However, it has been found that many
places in Northern China, Japan, Eastern Siberia, and Manchouria are
suitable for the purpose. The best southern stations for this method
lie unfortunately on the unexplored Antarctic continent and the islands
adjacent to it; but Crozet Island, Kerguelen Land, and some other
places more easy of access than the Antarctic continent, will serve
very well. Indeed, England has so many stations to occupy elsewhere
that it is doubtful whether she will care to undertake the dangerous
and difficult task of exploring the Antarctic wastes to secure the best
southern stations. The work may fairly be left to other nations, and
doubtless will be efficiently carried out.
What England will actually undertake has not yet been fully decided
upon. We may be quite certain that she will send out a party to Woahoo
or Hawaii to observe the accelerated commencement of the transit.
She will also send observers to watch the retarded commencement, but
whether to Crozet Island, Kerguelen Land, Mauritius, or Rodriguez is
uncertain. Possibly two parties will be sent out for this purpose, and
most likely Rodriguez and Mauritius will be the places selected. It
had been thought until lately that the sun would be too low at some of
the places when the transit begins, but a more exact calculation of
the circumstances of the transit has shown this to be a mistake. Both
Crozet Island and Kerguelen Land are very likely to be enveloped in
heavy mists when the transit begins—that is, soon after sunrise—hence
the choice of Mauritius and Rodriguez as the most suitable station.
England will also be called on to take an important part in observing
the accelerated end of the transit. A party will probably be sent to
Chatham Island or Campbell Island, not far from New Zealand. It had
been thought that at the former island the sun would be too low; but
here, again, a more exact consideration of the circumstances of the
transit has led astronomers to the conclusion that the sun will be
quite high enough at this station.
The Russian observers are principally concerned with the observation
of the retarded end of the transit, nearly all the best stations lying
in Siberia. But there are several stations in British India where
this phase can be very usefully observed; and doubtless the skilful
astronomers and mathematicians who are taking part in the survey of
India will be invited—as at the time of the great eclipse—to give their
services in the cause of science. Alexandria, also, though inferior
to several of the Indian stations, will probably be visited by an
observing party from England.
It will be seen that England will thus be called on to supply about
half-a-dozen expeditions to view the transit. All of these will be sent
out in pursuance of Delisle’s mode of utilising a transit, so that, for
reasons already referred to, it will be necessary that they should be
provided with instruments of the utmost delicacy, and very carefully
constructed.[4] They will have to remain at their several stations for
a long time before the transit takes place—several months, at least—so
that they may accurately determine the latitude of the temporary
observatories they will erect. This is a work requiring skilled
observers and recondite processes of calculation. Hence it is that the
cost of sending out these observing parties is so considerable.
The only English party which will apply Halley’s method of observation
is the one which will be stationed at Mauritius, under Lord Lindsay.
This part of their work will be comparatively easy, the method only
requiring that the duration of the transit should be carefully
timed. In fact, one of the great advantages of Halley’s method is
the smallness of the expense it involves. A party might land the day
before the transit, and sail away the day after, with results at
least as trustworthy as those which a party applying Delisle’s method
could obtain after several months of hard work. It is to this, rather
than any other cause, that the small expense of the observations
made in 1769 is to be referred. And doubtless had it been decided
by our astronomical authorities to apply Halley’s method solely or
principally, the expense of the transit-observations would have been
materially lessened. There would, however, have been a risk of failure
through the occurrence of bad weather at the critical stations; whereas
now—as other nations will doubtless avail themselves of Halley’s
method—the chance that the transit-observations will fail through
meteorological causes is very largely diminished. Science will owe much
to the generosity of England in this respect.
It is, indeed, only recently that the possibility of applying Halley’s
method has been recognised. It had been thought that the method must
fail totally in 1874. But on a more careful examination of the
circumstances of the transit, a French astronomer, M. Puiseux, was
enabled to announce that this is not the case. Almost simultaneously I
published calculations pointing to a similar result; but having carried
the processes a few steps further than M. Puiseux, I was able to show
that Halley’s method is not only available in 1874, but is the more
powerful method of the two.
Unfortunately, there is an element of doubt in the inquiry, of which
no amount of care on the part of our observers and mathematicians will
enable them to get rid. I refer to the behaviour of Venus herself. It
is to the peculiarity we are now to consider that the _quasi_-failure
of the observations made in 1769 must be attributed. It is true that
Mr. Stone, the first-assistant at the Greenwich Observatory, has
managed to remove the greater part of the doubts which clouded the
results of those observations. But not even his skill and patience can
serve to remove the blot which a century of doubt has seemed to throw
upon the most exact of the sciences. We shall now show how much of the
blame of that unfortunate century of doubt is to be ascribed to Venus.
At a transit, astronomers confine their attention to one particular
phase—the moment, namely, when Venus just seems to lie wholly within
the outline of the sun’s disc. This at least was what Halley and
Delisle both suggested as desirable. Unfortunately, Venus had not
been consulted, and when the time of the transit came she declined
to enter upon or leave the sun’s face in the manner suggested by the
astronomers. Consider, for example, her conduct when entering on the
sun’s face:—
At first, as the black disc of the planet gradually notched the edge of
the sun’s disc, all seemed going on well. But when somewhat more than
half of the planet was on the sun’s face, it began to be noticed that
Venus was losing her rotundity of figure. She became gradually more and
more pear-shaped, until at last she looked very much like a peg-top
touching with its point the edge of the sun’s disc. Then suddenly—‘as
by a lightning flash,’ said one observer—the top lost its peg, and then
gradually Venus recovered her figure, and the transit proceeded without
further change on her part until the time came for her to leave the
sun’s face, when similar peculiarities took place in a reversed order.
Here was a serious difficulty indeed. For when was the moment of true
contact? Was it when the peg-top figure seemed just to touch the edge
of the sun? This seemed unlikely, because a moment after the planet
was seen well removed from the sun’s edge. Was it when the rotund part
of the planet belonged to a figure which would have touched the sun’s
edge if the rotundity had been perfect elsewhere? This, again, seemed
unlikely, because at this moment the black band connecting Venus and
the sun was quite wide. And, besides, if this were the true moment of
contact, what eye could be trusted to determine the occurrence of a
relation so peculiar? Yet the interval between this phase and the final
or peg-top phase lasted several seconds—as many as twenty-two in one
instance in 1769—and the whole success of the observation depended on
exactness within three or four seconds at the outside.
We know that Venus will act in precisely the same manner in 1874. If
we had been induced to hope that improvements in our telescopes would
diminish the peculiarity, the observations of the transit of Mercury,
in November 1868, would have sufficed to destroy that hope, for even
with the all but perfect instruments of the Greenwich Observatory,
Mercury assumed the peg-top disguise in the most unpleasing manner.
It may be asked, then, What do astronomers propose to do in 1874 to
prevent Venus from misleading them again as she did in 1769? Much has
already been done towards this end. Mr. Stone undertook a series of
careful researches to determine the law according to which Venus may be
expected to behave, or to misbehave herself; and the result is, that he
has been able to tell the observers exactly what they will have to look
for, and exactly what it is most important that they should record. In
1769, observers recorded their observations in such doubtful terms,
owing to their ignorance of the real significance of the peculiarities
they witnessed, that the mathematicians who had to make use of those
observations were misled. _Hinc illæ lacrymæ._ Hence it is that an
undeserved reproach has fallen upon the ‘exact science.’
The amount of the error resulting from the misinterpretation of the
observations made in 1769 was, however, very small indeed, when its
true character is considered. It is, indeed, easy to make the error
seem enormous. The sun’s distance came out some four millions of miles
too large, and that seems no trifling error. Then, again, the resulting
estimate of the distance of Neptune came out more than a hundred
million miles too great; while even this enormous error was as nothing
when compared with that which resulted when the distances of the fixed
stars were considered.
But this is an altogether erroneous mode of estimating the effect of
the error. It would be as absurd to count up the number of hairs’
breadth by which the geographer’s estimates of the length and breadth
of England may be in error. In all such matters it is relative and not
absolute error we have to consider. A microscopist would have made a
bad mistake who should over-estimate the length of a fly’s proboscis
by a single hair’s breadth; but the astronomer had made a wonderfully
successful measurement of the sun’s distance who deduced it within
three or four millions of miles of the true value. For it is readily
calculable that the error in the estimated relative bearing of the sun
as seen from opposite sides of the earth corresponds to the angle which
a hair’s breadth subtends when seen from a distance of 125 feet.
The error was first detected when other modes of determining the sun’s
distance were applied by the skilful astronomers and physicists of
our own day. We have no space to describe as fully as they deserve
the ingenious processes by which the great problem has been attacked
without aid from Venus. Indeed, we can but barely mention the
principles on which those methods depend. But to the reader who takes
interest in astronomy, we can recommend no subject as better worth
studying than the masterly researches of Foucault, Leverrier, and
Hansen upon the problem of the sun’s distance.
The problem has been attacked in four several ways. First, the
tremendous velocity of light has been measured by an ingenious
arrangement of revolving mirrors; the result combined with the
known time occupied by light in travelling across the earth’s orbit
immediately gives the sun’s distance. Secondly, a certain irregularity
in the moon’s motion, due to the fact that she is most disturbed by the
sun when traversing that half of her path which is nearest to him, was
pressed into the service with similar results. Thirdly, an irregularity
in the earth’s motion, due to the fact that she circles around the
common centre of gravity of her own mass and the moon’s, was made a
means of attacking the problem. Lastly, Mars, a planet which, as we
have already mentioned, approaches us almost as nearly as Venus, was
found an efficient ally.
The result of calculations founded on these methods showed that the
sun’s distance, instead of being about 95,000,000 miles, is little
more than 91,500,000 miles. And recently a re-examination of the
observations made upon Venus in 1769 led Mr. Stone to believe that they
point to a similar result.
Doubtless, however, we must wait for the transit of Venus in 1874
before forming a final decision as to the estimate of the sun’s
distance which is to take its place in popular works on astronomy
during the next century or so. Nothing but an unlooked-for combination
of unfavourable circumstances can cause the failure of our hopes.
Certainly, if we should fail in obtaining satisfactory results in 1874,
the world will not say that the generosity of the English Government
has been in fault, since it would be difficult to find a parallel in
the history of modern science to the munificence of the grant which
has been made this year for expeditions to observe a phenomenon whose
interest and importance are purely scientific.
(From _St. Paul’s_, October 1869.)
FOOTNOTES:
[3] The reader will remember the time at which the essay appeared. For
several reasons it seems well to leave the essay unaltered. In the
second series of Light Science a later stage is presented, and the
account is carried up to the present date in my work on _The Transits
of Venus_.
[4] It is held to be of the utmost importance that all the observing
parties should use similar telescopes.
_BRITAIN’S COAL CELLARS._
It would have been deemed a strange thought in the days of the Tudors
to suggest that England’s greatness would one day depend,—or seem to
depend,—on her stores of coal, a mineral then regarded as only an
unpleasant rival of the wood-log for household fires. When Shakespeare
put into the mouth of Faulconbridge the words—
This England never did, nor never shall,
Lie at the proud foot of a conqueror,
But when it first did help to wound itself,
he would have thought it a singular proviso that England should be
watchful of her coal stores if she would preserve her position among
the nations. And yet there is a closer connection between the present
greatness of Britain and the mighty coal cellars underlying certain
British counties than we are commonly prepared to acknowledge. Saxon
steadiness and Norman energy have doubtless played their part in
placing Britain in the position she now holds; but whatever may have
been the case in past ages of our history, it is certain that at
present there is much truth in Liebig’s assertion that England’s power
is in her coal. The time may come again, as the time has been, when we
shall be less dependent on our coal stores,—when bituminous bankruptcy
will not be equivalent to national bankruptcy; but if all our coal
mines were at this moment rendered unworkable, the power of England
would receive a shock from which it would be ages in recovering.
I have quoted an assertion made many years since by Baron Liebig. The
assertion was accompanied by another not less striking. ‘Civilisation,’
he said, ‘is the economy of power; and English power is coal.’ It is
on this text that I propose now to comment. There has recently been
issued a Blue Book, bearing in the most important manner on the subject
of England’s coal-supply. For five years fifteen eminent Commissioners
have been engaged in examining the available evidence respecting the
stores of coal contained in the various coal-fields of Great Britain.
Their inquiries were commenced soon after the time when the fears of
the country on this subject were first seriously awakened; and were
directed specially to ascertain how far those fears were justified
by the real circumstances of the case. It will be well to compare
the various opinions which were expressed before the inquiries were
commenced, with the results which have now been obtained.
In the first place it should be noticed that the subject had attracted
the attention of men of science many years ago. Some forty years[5]
have passed since Dr. Buckland, in one of the Bridgewater Treatises,
pointed to the necessity for a careful examination of our coal stores,
lest England should drift unawares into what he called ‘bituminous
bankruptcy.’ At that time the quantity of coal raised annually in
England amounted to but about forty millions of tons. Ten years later
the annual yield had risen to about fifty millions of tons; and then
another warning voice was raised by Dr. Arnold. Ten more years passed,
and the annual yield had increased to 83,635,214 tons, when Mr. Hull
made the startling announcement that our coal stores would last us but
about two centuries, unless some means were adopted to check the lavish
expenditure of our black diamonds.
But it was undoubtedly the address of Sir W. Armstrong to the British
Association, in 1863, which first roused the attention of the country
to the importance of the subject. ‘The greatness of England,’ he said,
‘depends much upon the superiority of her coal, in cheapness and
quality, over that of other nations. But we have already drawn from our
choicest mines a far larger quantity of coal than has been raised in
all other parts of the world put together; and the time is not remote
when we shall have to encounter the disadvantages of increased cost
of working and diminished value of produce.’ Then he summed up the
state of the case as he viewed it. ‘The entire quantity of available
coal existing in these islands has been calculated to amount to 80,000
millions of tons, which, at the present rate of consumption, would be
exhausted in 930 years; but with a continued yearly increase of 2¾
millions of tons would only last 212 years.’
Other statements were not wanting, however, which presented matters in
a more favourable light. Mr. Hussey Vivian, M.P., expressed the opinion
that South Wales alone could supply all England with coals for 500
years. Mr. R. C. Taylor, of the Geological Society, said that our coal
stores would suffice for 1,700 years. And there were some who adopted a
yet more sanguine view of our position.
On the other hand, Mr. Edward Hull, of the Geological Survey,
calculated that with an increase of but one million and a half of tons
per annum—considerably less than even the average increase for the
preceding decade[6]—our coals would last us but a little more than
300 years. Mr. Stanley Jevons, in his masterly treatise on ‘The Coal
Question,’ adopted a mode of considering the increase, which has led
to an even more unpleasant conclusion than any hitherto obtained. He
observed that the quantity of coal raised in successive years is not
merely increasing, but the amount of increase is itself increasing.
‘We, of course, regard not,’ he said, ‘the average annual arithmetical
increase of coal consumption between 1854 and 1863, which is 2,403,424
tons, but the average rate per cent. of increase, which is found by
computation to be 3·26 per cent.’ That is to say, for every hundred
tons of coal consumed in one year, 103¼ tons, or thereabouts, would
be consumed in the next—taking one year with another. Without entering
into technicalities, or niceties of calculation, it is easy to show
the difference between this view of the matter and a view founded only
on the average increase during so many years. Consider 10,000 tons of
coal sold in one year, then Mr. Stanley Jevons points out that instead
of that amount, 10,326 would be sold in the next; and so far we may
suppose that the other view would agree with his. But in the next, or
third year (always remembering, however, that we must take one year
with another), the increase of 326 tons would not be merely doubled,
according to Mr. Stanley Jevons; that is, the consumption would not be
only 10,652 tons:—the 10,000 of the second year would be replaced by
10,326 tons in the third year, and the remaining 326 would be increased
by 3¼ tons for each hundred, or by rather more than 10½ tons;
so that in all there would be 10,662¼ tons, instead of 10,652.
Now the difference in this third year seems small, though when it is
applied to about nine thousand times 10,000 tons it is by no means
small, amounting in fact to 95,000 tons; but when the principle is
extended to sequent years its effects assume paramount importance. The
small increase is as the small increase of a farthing for the second
horseshoe-nail in the well-known problem. The effects, after a few
years have passed, correspond to the thousands of pounds by which the
last shoe-nails of that problem increase the cost of the horse. As Mr.
Leonard Lemoran points out in the paper mentioned in the above note,
if the assumed rate per cent. of increase continue, ‘we should draw in
the year 1900 from our rocks more than 300 millions of tons, and in
1950 more than 2,000 millions.[7] About 300,000 miners are now (1866)
employed in raising rather more than 92 millions of tons of coals;
therefore more than eight million miners would be necessary to raise
the quantity estimated as the produce of 1950. One-third of the present
population of Great Britain would be coal miners.’ Or as Mr. Jevons
himself sums up our future, ‘If our consumption of coal continue to
multiply for 110 years at the same rate as hitherto, the total amount
of coal consumed in the interval would be 100,000 millions of tons.’
Now as Mr. Hull estimated the available coal in Great Britain, within
a depth of 4,000 feet, at 83,000 millions of tons, it followed that,
adopting Mr. Jevons’s mode of calculation, a century would exhaust
‘all the coal in our present workings, as well as all the coal seams
which may be found at a depth of 1,500 feet below the deepest working
in the kingdom.’ It should be added, however, that Mr. Stanley Jevons
mentioned 200,000 millions of tons as the probable limit of the coal
supplies of Great Britain.
The opinion of Mr. Jevons respecting the probable rate of increase
of our consumption was not accepted by the generality of those who
examined the subject in 1865 and 1866. There were some, indeed, who
considered that the assumption was ‘absurd in every point of view.’ In
one sense, indeed, Mr. Jevons himself would have been ready to admit
that his estimates would not be justified by the result. The observed
rate of increase could not possibly be maintained beyond a certain
epoch, simply because there would not be enough men to work the coal
mines to the extent required. But, regarding the increase as indicating
the requirements of the kingdom, it would matter little whether the
necessary supply failed for want of coal or for want of the means of
raising the coal. In other words, removing the question from the arena
of geological dispute, and considering only the requirements of the
country, we should have this disagreeable conclusion forced upon us,
if Mr. Jevons’s estimate is just, that England will not be able, a
century, or even half a century hence, to get as many coals from her
subterranean cellars as she will then require. She may have the coals,
but she will not have men enough to bring them to bank.
It is, perhaps, in this aspect, that the question assumes its chief
interest for us. Rightly understood, the statements of Mr. Jevons
were of vital importance; so important, indeed, that the nation
might have looked forward to the results of the Commission much as
a patient would await the physician’s report of the result of a
stethoscopic examination. The power of the nation residing—for the
nonce at least—in her coal, the enforced consumption of coal at a
rate which cannot be maintained (from whatever cause), means to all
intents and purposes the decline and approaching demise of England’s
power as a nation. Furthermore, apart from all inquiries such as the
Commissioners undertook to make, the mere statement of the successive
annual yields was to be looked upon as of vital interest, precisely as
the progressive waste of a consumptive patient’s strength and substance
suggests even more serious apprehensions than the opinion of the
physician.
I have said that many eminent authorities held that the rate of
increase assumed by Mr. Jevons would not actually prevail. But some
went farther, and questioned whether the average annual arithmetical
increase of the lately passed years would continue even for the next
few years after the publication of Mr. Jevons’s work. ‘Such a continued
increase as that, which has taken place during the last five years,’
wrote an excellent practical authority, ‘cannot continue for the next
ten years,’—far less, therefore, that increasing rate of increase which
Mr. Jevons had assumed. The same writer went farther even than this.
For, after pointing out that the exportation of coal would probably
be soon reduced, rather than undergo, as during the past, a steady
increase, he added that ‘on every side there were evidences of the
most decided character, warranting the supposition that the annual
exhaustion of our coal fields would not at any period much exceed the
hundred million tons which it had nearly reached’ (in 1866).
One of the most interesting questions, then, which the Commissioners
were called upon to decide was, whether, at least during the period of
their labours, the anticipations of Mr. Jevons would be fulfilled or
not. It is easy to compare his anticipations with those above quoted;
or rather, it is easy to determine whether Mr. Jevons’s theory of an
increasing increase, or the theory of a uniform average increase,
accords best with the experience of the last five years. To make the
comparison fairly we must adopt the figures on which his own estimate
was founded. We have seen that he rejected the annual increase of
2,403,424 deduced from the records of the nine preceding years, and
adopted instead an increase of 3¼ per cent. year by year, taking
one year with another. His own calculations gave for this year 1871
a consumption of 118 millions of tons,—an enormous increase on the
annual consumption when he wrote. According to the view he rejected,
the consumption for the year 1871 is easily computed, though slightly
different results will be obtained, according to the year we choose
to count from. The annual increase above mentioned gives an increase
of 24,034,240 tons in ten years, and if we add this amount to the
consumption in 1861 (83,635,214 tons) we obtain for the year 1871 a
consumption of 107,669,454 tons. On the other hand, if we add eight
years’ increase to the consumption of 1863 (88,292,515 tons), we
obtain 107,519,907 tons.[8] It will be seen that there is an important
difference between the consumption for 1871, as estimated according to
Mr. Jevons’s view, and according to the average rate of increase in the
nine preceding years. As the matter stood in 1865, the great question
concerning the consumption of the year 1871 would have been,—whether it
would be nearer 118 millions, the estimate of Mr. Jevons; or to 107½
millions, the estimate, according to the annual rate of increase;
or, lastly, to a number of tons, not much, if at all, exceeding 100
millions?
The answer of the Commissioners comes in no doubtful terms. Judging
from the consumption during the four years ending in 1870, the
estimated consumption for the year 1872 is no less than 115 millions,
an amount approaching Mr. Jevons’s estimate much more nearly than
could be desired. Indeed, if we consider the imperfect nature of the
statistics on which he founded his calculations, the agreement between
his estimate and the observed result must be regarded as surprisingly
close. Remembering the conclusion to which Mr. Jevons came with respect
to the period for which our coal stores would last, and noticing the
close agreement thus far between his anticipations and the result,
we can well understand the warning tone of the report issued by the
Commissioners. ‘Every hypothesis,’ they say, ‘must be speculative, but
it is certain that if the present rate of increase in the consumption
of coal be indefinitely continued, even in an approximate degree, the
progress towards the exhaustion of our coal will be very rapid.’ Let
it be remembered that the Commission was issued at the instance of
those who took the more sanguine view, and that it included within
its ranks such eminent authorities as Sir William Armstrong, Sir
Robert Murchison, Professor Ramsay, Mr. John Hunt, and others of like
experience in the subject under inquiry.
If, in the next place, we compare Mr. Jevons’s estimate of the
quantity of coal available for use with the result obtained by the
Commissioners, we find little to restore our confidence in the extent
of time during which our coal stores may be expected to last. We have
seen that 200,000 millions of tons had been supposed to be available;
but the Commissioners find that ‘we now have an aggregate of 146,480
millions of tons, which may be reasonably expected to be available
for use.’ Again, it had been supposed that our coal mines could be
worked to a depth of 4,000 feet, or to an even greater depth. ‘The
difficulties in the way of deep mining,’ wrote Mr. Lemoran in 1866,
‘are mere questions of cost. It is important to notice that the
assumption of 4,000 feet as the greatest depth to which coal can be
worked, on account of the increase of temperature, is purely voluntary.
The increase has been calculated at a rate for which there is no
authority; and while we are saying our coal-beds cannot be worked below
4,000 feet, a colliery in Belgium has nearly approached that depth, and
no inconvenience is experienced by the miners.’ But the Commissioners
state that at a depth of only 2,419 feet in the Rosebridge mine (the
deepest in England), the temperature is 94 degrees of Fahrenheit, or
within four degrees of blood heat. ‘The depth at which the temperature
of the earth would amount to blood heat,’ they add, ‘is about 3,000
feet.’ They express a belief that by the ‘long wall system’ of working
(a system as yet seldom adopted in the chief northern mines) it will be
possible to reach a depth of 3,420 feet before this heat is attained;
but it is by no means certain that this will prove to be the case.
On the other hand, it will be well to regard the more promising aspect
of the question.
We must not forget, in the first place, that in all matters of
statistical research there is room for misapprehension unless careful
attention be paid, not merely to the observed facts, but to the
circumstances with which those facts are more or less intimately
associated. If we consider, for example, the progress of the
consumption of our coal during the past fifteen years, we find that a
law of increase exists, which is, as we have seen, easily expressed,
and which, after being tested by a process resembling prediction, has
been singularly confirmed by the result. But if we inquire into the
various causes of the great increase in the consumption of coals, we
find that while those causes have been increasing in activity—so to
speak—to a degree quite sufficient to explain the observed consumption,
they are yet such as in their very nature must needs be unable to
pass beyond a certain range of increase. Thus the population of Great
Britain has been steadily increasing, and at present the annual
increase is itself increasing. Then the amount of coal used in inland
communication is increasing, not only on account of the gradual
extension of the railway network, but also on account of the increase
of population, of commerce, and so on. Again, our commerce with other
countries has increased with great rapidity since the year 1860, when
the French treaty came into operation, and it will continue to increase
with the increase of our population, of our means of communication
within our own country as well as with foreign countries, and so on.
But all these causes of increase are now growing in activity at a rate
which must inevitably diminish. Our population cannot increase beyond a
certain extent, because the extent of the country will suffice for but
a certain number of inhabitants. If emigration do not prevent increase
beyond that number, other causes will, or else a much more serious evil
than the exhaustion of all our coal stores awaits the country. Again,
the requirements of inland communication will before long be so far
met that no such rapid extension as is now in progress will be called
for. After convenient communication has been established between all
parts of the country—whether the process require the formation of new
lines or of new services—no important increase can be required. As
regards our commerce, its increase depends necessarily on the increase
at present going on in the requirements of the country. Year by year
Britain has a larger population, and the average requirements of each
member of the population are also increasing. But we have seen that
the increase of her population is necessarily limited; and although
the increase of the requirements of her people may not be (strictly
speaking) limited, yet it is manifest that, inasmuch as that increase
depends on causes which are themselves approaching a limit, its rate
must, after a time, continually diminish. Let it be understood that,
when I speak of the requirements of the population, I do not mean
only what they must obtain from other countries. The commerce of a
country is the expression of the activity with which the nation is
‘earning its living,’ so to speak, and in a given population there is
a limit to what is necessary for this purpose, precisely as there is
a limit to the sum which an individual person in any given state of
life requires for the maintenance of a given family. Indeed, although
such comparisons are not always safe, we may in this case compare
what may be called the commercial requirements of the nation with the
requirements of the head of the family,—a merchant suppose. There are
no limits to the degree of wealth which a merchant may _desire_ to
gain, but unquestionably there are limits to the income necessary to
maintain his house and family and mercantile position. Supposing he
were extending his gains far beyond his actual requirements, it would
by no means imply his approaching ruin that there was a demonstrable
limit to this extension. And in like manner, it would seem that, apart
from the limits set by nature to the extension of our population,
it need by no means be assumed that if our commerce showed signs of
approaching a limit, the downfall of England’s power would be at hand.
In fact, we cannot accept Mr. Jevons’s figures for distant epochs
without first inquiring whether it is likely that at those epochs the
circumstances on which the consumption of our coal depends will be
correspondingly changed. Supposing that 120 millions of tons of coals
suffice for the requirements of our present population, we can scarcely
believe that 1,440 millions will be needed in 1950, unless we suppose
that the population of Britain will be twelve times greater than at
present; or that the population will be even greater than this, since
the consumption of coal upon our railways could scarcely be expected
to increase in proportion to the population. Now no one believes that
Britain will number 300 millions of inhabitants in 1950, or in 2950;
the country could not maintain half that number, even though all her
available stores of coal and iron, and other sources of commercial
wealth were increased a hundredfold.
It is a mistake, indeed, to extend the results of statistical research
very far beyond the time to which the facts and figures belong. It
would be easy to multiply instances of the incorrectness of such a
process. To take a single case.—When cholera has been extending its
ravages in this country, the statistics of mortality from that cause,
if studied with reference to four or five successive weeks, have
indicated a law of increase, which is very readily expressed so as
to accord with the mortality during those weeks, and perhaps two or
three following weeks. But if such a law were extended indefinitely it
might be found to imply nothing short of the complete desolation of
the country by cholera, within the space of a few months. Thus, if the
deaths (from cholera) in five successive weeks were 20, 27, 35, 47, and
63,—numbers corresponding with the general characteristics of cholera
mortality in the earlier stages of a visitation,—the weekly mortality a
year later, estimated according to the observed percentage of increase,
would be more than 173 millions! Now this method of estimation, though
leading to this preposterous conclusion as respects a more distant
epoch, would probably lead to tolerably correct results for the next
week or two after that in which 63 persons died,—the estimated numbers
being 84 and 110 for the next two weeks respectively.
It seems to me, therefore, that we are not justified, by the observed
seeming fulfilment of Mr. Jevons’s anticipations, in concluding that
a hundred years hence the consumption of coals will be 2,000 millions
of tons, or that the total consumption during the next 110 years will
be 100,000 millions of tons. We might almost as safely infer that
because a growing lad requires such and such an increase of food year
by year, the grown man will need a similar rate of increase, and the
septuagenarian require so many tons and hogsheads of solid and liquid
food _per diem_.
At present it does not seem possible to arrive at any definite
conclusions respecting the probable consumption of coal in years to
come. The range of observation is not sufficiently extended. It seems
clear, indeed, that the epoch is not near at hand when the present law
of increase will be modified. This is shown by the agreement of the
observed results during the past five years with the anticipations of
Mr. Jevons. It would be altogether unsafe to predict that the yearly
consumption will not rise to 150 or 200 or even 250 millions of tons
per annum, or to point to any definite stage at which the present
increasing rate of increase will be changed first into uniform (or
arithmetical) increase, and thence into a decreasing rate of increase.
But it appears to me that no question can exist that these changes
_will_ take place. We might even go farther, and regard it as all but
certain that the time will come when there will be no annual increase.
Nay, unless the history of this country is to differ from the history
of all other nations which have attained to great power, the time
might be expected to arrive when there will be, year by year, a slow
diminution in the commercial activity of Britain, and a corresponding
diminution in the exhaustion of her coal stores. There is room for
an amazing increase in Britain’s power and greatness, room also for
an unprecedented continuance of these attributes, while yet the coal
stores of the country remain well supplied.
Let us conceive, for instance, that the greatest annual consumption
of coal during the future years of England’s existence as a great
nation, should be set at three times her present annual consumption,
or at 350 millions of tons. Few will regard this as an unduly low
estimate when they remember that it is exceedingly unlikely that the
present population of Britain will ever be tripled, and that a triple
population could be commercially far more active (in relation to its
numbers) than the present population, with no greater consumption of
coal per head. Now, to begin with, if this enormous annual consumption
began immediately, we should yet (with Mr. Jevons’s assumption as to
the quantity of available coal) have 570 years’ lease of power instead
of 110. But, as a matter of fact, so soon as we have recognised the
principle that there is a limit to the increase of annual consumption,
we are compelled to believe that that limit will be approached by
a much gentler gradient, so to speak, than the same consumption as
attained on Mr. Jevons’s assumption. According to his view, in fact, an
annual consumption of 350 millions of tons per annum will be attained
early in the twentieth century; but according to the theory which
sets such a consumption as the highest ever to be attained, we should
place its attainment several hundreds of years later. This is a vague
statement, I admit, but the very fact on which I am mainly insisting
is this, that the evidence at present in our hands is insufficient as
a basis of exact calculation. Now, if we set 500 years hence as the
time when the annual consumption of coal will have reached the above
enormous amount, we should set the total consumption during those
centuries at about one-half that due to an annual consumption of 350
millions of tons. In that case there would still remain coal enough to
supply the country for 320 years at the same tremendous rate. In all,
on these suppositions, 820 years would be provided for. These would
be years of commercial activity far exceeding that of our own day—in
fact, they would be years during which Britain would be accumulating
wealth at a rate so enormous that at the end of the era she would be
not wholly unprovided with the means of supporting her existence as a
nation, apart from all reference to her mineral stores. It is, indeed,
utterly inconceivable, I think, that Great Britain and her people will
ever be _able_ to progress at the rate implied by these suggestions.
To conceive of Great Britain as arriving at ruin within a thousand
years by the over-rapid exhaustion of her coal stores, is, in fact,
equivalent to supposing that she will attain in the interval to a
wholly unprecedented—I had almost said a wholly incredible—degree of
wealth and power.
As regards the evidence which has been adduced respecting the extent
of the available coal supply, it is to be remarked that, on the whole,
the result cannot be regarded as unfavourable. The more sanguine
views entertained five or six years ago have not, indeed,, been fully
justified. Yet our coal supply has been shown to be enormous, even when
considered with reference to the continually increasing exhaustion.
But it must be admitted that the question of the depth to which
our coal mines may be conveniently or even possibly worked, has an
unpleasantly doubtful aspect. Of the stores which the Commissioners
regard as available a vast proportion must be mined out from depths
far exceeding any which have been at present reached in England. It is
not as yet clear how far the increase of depth will add to the cost
and risk of working; nor do I propose to discuss a subject which can
only be adequately dealt with by those who possess practical knowledge
of the details of colliery-working. I will content myself by quoting
some remarks on the subject, in an inaugural address delivered by Mr.
George Elliot (one of the Royal Commissioners) before the North of
England Institute of Mining Engineers in 1868. ‘The great depth,’ he
remarked, ‘at which many of our pits are worked, and the vast extent
of their lateral ramifications, make it more than ever necessary
that we should secure the best mode of rendering the supply of pure
air certain, regular, and safe. It is maintained that ventilating
by machinery ensures these desiderata; that the nicety with which
mechanical appliances may be regulated, the delicate adjustment of
power of which they are capable, and the complete safety with which
they may be worked, place them far before the system they are intended
to supersede. The extent of our coal supply will be materially
increased by the improvement of which this is a type.... It is probable
that the ordinary means of ventilation—whether by furnace or fan—may
be aided by a change in the force or agency employed for the purposes
of haulage and other independent work. As an instance of my meaning, I
may mention that the apparatus which I have introduced in South Wales,
and which, by means of compressed air used as a motive power instead of
steam, draws trams and pumps water with complete success, is found to
generate ice in an atmosphere which is naturally hot and oppressive.
The mechanical usefulness of these new air-engines seems capable
of indefinite extension; while, as their cooling properties form a
collateral advantage arising out of their use, it is at least possible
that they may prove valuable auxiliaries to the more regular means of
ventilation in extending the security and promoting the healthfulness
of our mines. _The difficulties of ventilation once surmounted, the
extent of coal at our disposal is incalculably increased._‘
In the address just quoted there are some striking suggestions as
to the possibility of working those coal fields which extend below
the sea on our east and west coasts, especially in the counties of
Durham, Northumberland, and Cumberland. Mr. Elliot remarks that ‘for
all practical purposes these fields are as entirely within the reach
of the mining engineer as the ordinary workings out of which coal is
hewed.’ It is known that in many districts the coal strata extend ten
or twelve miles beyond the shore; and Mr. Elliot believes that by
sinking ventilating shafts in the German Ocean the coal below may be
safely worked. The idea seems somewhat daring; yet, after the feats
of engineering which have been achieved in our day, there seems no
valid reason for doubting that at least when the pressure of a failing
coal supply begins to be felt, the means will be found for rendering
these immense submarine coal stores available. As to the difficulty
of transport, Mr. Elliot remarks that, according to his estimates,
‘transport would neither be more costly nor more laborious than it has
been in days gone by to convey coal the same distance after it was
brought to the surface inland.’ The enormous importance of the subject
is shown by the fact that ‘out of the minerals obtainable in Durham
alone, one-third,’ Mr. Elliot tells us, ‘may be held to lie under the
sea, and that all coalfields having a similar inclination of strata,
and bordering on the ocean, will be similarly enlarged. This at once
disposes,’ he adds, ‘of some of the fears expressed as to the duration
of our coal supply; and while I am quite aware that these theories may
be challenged, they are not put forward without due deliberation, and I
am content to stake my professional reputation on their practicability.’
With regard to the future of this country, it appears to me that
little anxiety need be entertained. Apart from the considerations I
have urged, which seem to indicate that our consumption cannot long
increase at the same rate as at present, it seems not unreasonable to
anticipate that within the next few decades science will find the means
of economising our coals in more ways than one. It does not indeed
appear likely that any form of fuel will ever take the place of coal;
but a portion of the work now derived from the consumption of coal
may be expected to be derived in future years from some of the other
substances now coming into use. It may be hoped, also, that science may
suggest means for bringing coals to the surface with less waste, and
even at less cost, than at present. And in other ways the process of
exhaustion may be more or less effectively checked.
But while we may thus look somewhat confidently forward, as I judge,
to the future of our country, serious questions are suggested as
to the future of the human race. The period during which a nation
flourishes, long as it seems by comparison with the life of man, yet
sinks into insignificance when compared with the period during which
civilised men will bear sway upon the earth. The thousands of years
during which the coal stores of the earth may be expected to last will
pass away, and then the descendants of those now living on the earth
will have to trust to other force-supplies than those which we are now
using so lavishly. It may seem fanciful to look so far forward, and
yet by comparison with the periods which the astronomer deals with
in considering the future of our earth, thousands of years are as
nothing. As I have said elsewhere, ‘those thousands of years will pass
as surely as the thousands which have already passed, and the wants
entailed by wastefulness in our day will then be felt, and none the
less that for so many years there had been no failure in the supplies
contained within the great subterranean storehouse.’ It behoves us to
consider thoughtfully the wants even of those distant eras. If the
greatest good for the greatest number is to be regarded as the true
rule for the conduct of intelligent beings, then unquestionably mere
distance in point of time should not prevent us from anticipating the
requirements of those remote descendants of ours. We should regard the
consciousness of this duty and its performance as signs by which the
superiority of our own over less civilised times is partly manifested.
As man is in dignity higher than non-intelligent animals, in that he
alone provides of his own forethought for the wants of his children, so
our generation would be raised in dignity above preceding generations
if it took intelligent charge of the wants of its remote descendants.
We ourselves are now employing stores of force laid up for us by the
unconscious processes of Nature in long past ages. As Professor Tyndall
has finely said, we are utilising the Sun of the Carboniferous Epoch.
The light ‘which streamed earthwards from the sun’ was stored up for
us by the unconscious activity of ‘organisms which living took into
them the solar light, and by the consumption of its energy incessantly
generated chemical forces.’ The vegetable world of that old epoch
‘constituted the reservoir in which the fugitive solar rays were fixed,
suitably deposited, and rendered ready for useful application.’ What
the vegetable world did for us unconsciously during the Carboniferous
Epoch, the scientific world of our epoch must do for our remote
descendants. While we are consuming the stores of force laid up in past
ages for our benefit, we must invent the means for obtaining directly
from the solar rays fresh and inexhaustible supplies of motive energy.
(From the _St. Paul’s Magazine_, November 1871.)
_THE SECRET OF THE NORTH POLE._
If an astronomer upon some distant planet has ever thought the tiny
orb we inhabit worthy of telescopic study, there can be little doubt
that the snowy regions which surround the arctic and antarctic poles
must have attracted a large share of his attention. Waxing and waning
with the passing seasons, those two white patches afford significant
information respecting the circumstances of our planet’s constitution.
They mark the direction of the imaginary axial line upon which the
planet rotates; so that we can imagine that an astronomer on Mars or
Venus would judge from their position how it fares with terrestrial
creatures. There may, indeed, be Martial Whewells who laugh to scorn
the notion that a globe so inconveniently circumstanced as ours can
be inhabited, and are ready to show that, if there were living beings
here, they must be quickly destroyed by excessive heat. On the other
hand, there are possibly sceptics on Venus also who smile at the vanity
of those who can conceive a frozen world, such as this our outer
planet, to be inhabited by any sort of living creature. But we doubt
not that the more advanced thinkers both in Mars and Venus are ready
to admit that, though we must necessarily be far inferior beings to
themselves, we yet manage to ‘live and move and have our being’ on this
ill-conditioned globe of ours. And these, observing the earth’s polar
snow-caps, must be led to several important conclusions respecting
physical relations here.
It is, indeed, rather a singular fact to contemplate, that
ex-terrestrial observers, such as these, may know much more than we
ourselves do respecting those mysterious regions which lie close around
the two poles. Their eyes may have rested on spots which, with all
our endeavours, we have hitherto failed to reach. Whether, as some
have thought, the arctic pole is in summer surrounded by a wide and
tide-swayed ocean; whether there lies around the antarctic pole a wide
continent bespread with volcanic mountains larger and more energetic
than the two burning cones which Ross found on the outskirts of this
desolate region; or whether the habitudes prevailing near either
pole are wholly different from those suggested by geographers and
voyagers—such questions as these might possibly, be resolved at once,
could our astronomers take their stand on some neighbouring planet, and
direct the searching power of their telescopes upon this terrestrial
orb. For this is one of those cases referred to by Humboldt, when he
said that there are circumstances under which man is able to learn more
respecting objects millions of miles away from him than respecting the
very globe which he inhabits.
If we take a terrestrial globe, and examine the actual region near
the North Pole which has as yet remained unvisited by man, it will
be found to be far smaller than many imagine. In nearly all maps
the requirements of charting result in a considerable exaggeration
of the polar regions. This is the case in the ordinary ‘maps of the
two hemispheres’ which are to be found in all atlases. And it is, of
course, the case to a much more remarkable extent in what is termed
Mercator’s projection. In a Mercator’s chart we see Greenland, for
example, exaggerated into a continent fully as large as South America,
or to seven or eight times its real dimensions.
There are three principal directions in which explorers have attempted
to approach the North Pole. The first is that by way of the sea which
lies between Greenland and Spitzbergen. I include under this head Sir
Edward Parry’s attempt to reach the pole by crossing the ice-fields
which lie to the north of Spitzbergen. The second is that by way of the
straits which lie to the west of Greenland. The third is that pursued
by Russian explorers who have attempted to cross the frozen seas which
surround the northern shores of Siberia.
In considering the limits of the unknown north-polar regions, we shall
also have to take into account the voyages which have been made around
the northern shores of the American continent in the search for a
‘north-west passage.’ The explorers who set out upon this search found
themselves gradually forced to seek higher and higher latitudes in
order to find a way round the complicated barriers presented by the
ice-bound straits and islands which lie to the north of the American
continent. And it may be noticed in passing, as a remarkable and
unforeseen circumstance, that the farther north the voyagers went the
less severe was the cold they had to encounter. We shall see that this
circumstance has an important bearing on the considerations I shall
presently have to deal with.
One other circumstance respecting the search for the north-west
passage, though not connected very closely with my subject, is so
singular and so little known that I feel tempted to make mention of it
at this point. The notion with which the seekers after a north-west
passage set out was simply this, that the easiest way of reaching
China and the East Indies was to pursue a course resembling as near as
possible that on which Columbus had set out—if only it should appear
that no impassable barriers rendered such a course impracticable. They
quickly found that the American continent presents an unbroken line
of land from high northern latitudes far away towards the antarctic
seas. But it is a circumstance worth noticing, that if the American
continents had no existence, the direct westerly course pursued
by Columbus was not only not the nearest way to the East Indian
Archipelago, but was one of the longest routes which could possibly
have been selected. Surprising as it may seem at first sight, a voyager
from Spain for China and the East Indies ought, if he sought the
absolutely shortest path, to set out on an almost direct northerly
route! He would pass close by Ireland and Iceland, and onwards past the
North Pole into the Pacific. This is what is called the great-circle
route; and if it were only practicable one, would shorten the journey
to China by many hundreds of miles.
Let us return, however, to the consideration of the information which
arctic voyagers have brought us concerning the north-polar regions.
The most laborious researches in arctic seas are those which have
been carried out by the searchers after a north-west passage. I shall
therefore first consider the limits of the unknown region in this
direction. Afterwards we can examine the results of those voyages which
have been undertaken with the express purpose of reaching the North
Pole along the three principal routes already mentioned.
If we examine a map of North America constructed in recent times, we
shall find that between Greenland and Canada an immense extent of
coast-line has been charted. A vast archipelago covers this part of
the northern world. Or, if the strangely-complicated coastlines which
have been laid down really belong to but a small number of islands,
the figures of those must be of the most fantastic kind. Towards the
north-west, however, we find several islands whose outlines have been
entirely ascertained. Thus we have in succession North Devon Island,
Cornwallis Island, Melville Island, and Port Patrick Island, all lying
north of the seventy-fifth parallel of latitude. But we are not to
suppose that these islands limit the extent of our seamen’s researches
in this direction. Far to the northward of Wellington Channel, Captain
de Haven saw, in 1852, the signs of an open sea—in other words, he
saw, beyond the ice-fields, what arctic seamen call a ‘water sky.’ In
1855 Captain Penny sailed upon this open sea; but how far it extends
towards the North Pole has not yet been ascertained.
It must not be forgotten that the north-west passage has been shown
to be a reality, by means of voyages from the Pacific as well as from
the Atlantic. No arctic voyager, however, has yet succeeded in passing
from one ocean to the other. Nor is it likely now that any voyager will
pursue his way along a path so beset by dangers as that which is called
the north-west passage. Long before the problem had been solved, it had
become well known that no profit could be expected to accrue to trade
from the discovery of a passage along the perilous straits and the
ice-encumbered seas which, lie to the north of the American continent.
But Sir Edward Parry having traced out a passage as far as Melville
Island, it seemed to the bold spirit of our arctic explorers that it
might be possible, by sailing through Behring’s Straits, to trace out a
connection between the arctic seas on that side and the regions reached
by Parry. Accordingly, M’Clure, in 1850, sailed in the ‘Investigator,’
and passing eastward, after traversing Behring’s Straits, reached
Baring’s Land, and eventually identified this land as a portion of
Banks’ Land, seen by Parry to the southward of Melville Island.
It will thus be seen that the unexplored parts of the arctic regions
are limited in this direction by sufficiently high latitudes.
Turn we next to the explorations which Russian voyagers have made
to the northward of Siberia. It must be noticed, in the first place,
that the coast of Siberia runs much farther northward than that of
the American continent. So that on this side, independently of sea
explorations, the unknown arctic regions are limited within very high
latitudes. But attempts have been made to push much farther north from
these shores. In every case, however, the voyagers have found that
the ice-fields, over which they hoped to make their way, have become
gradually less and less firm, until at length no doubt could remain
that there lay an open sea beyond them. How far that sea may extend
is a part of the secret of the North Pole; but we may assume that
it is no narrow sea, since otherwise there can be little doubt that
the ice-fields which surround the shores of Northern Siberia would
extend unbroken to the farther shores of what we should thus have to
recognise as a strait. The thinning-off of these ice-fields, observed
by Baron Wrangel and his companions, affords, indeed, most remarkable
and significant testimony respecting the nature of the sea which lies
beyond. This I shall presently have to exhibit more at length; in the
meantime I need only remark that scarcely any doubt can exist that
the sea thus discovered extends northwards to at least the eightieth
parallel of latitude.
We may say, then, that from Wellington Channel, northward of
the American continent, right round towards the west, up to the
neighbourhood of Spitzbergen, very little doubt exists as to the
general characteristics of the arctic regions, save only as respects
those unexplored parts which lie within ten or twelve degrees of
the North Pole. The reader will see presently why I am so careful
to exhibit the limited extent of the unexplored arctic regions in
this direction. The guess we shall form as to the true nature of the
north-polar secret will depend almost entirely on this consideration.
I turn now to those two paths along which arctic exploration, properly
so termed, has been most successfully pursued.
It is chiefly to the expeditions of Drs. Kane and Hayes that we owe the
important knowledge we have respecting the northerly portions of the
straits which lie to the west of Greenland. Each of these explorers
succeeded in reaching the shores of an open sea lying to the north-east
of Kennedy Channel, the extreme northerly limit of those straits.
Hayes, who had accompanied Kane in the voyage of 1854-5, succeeded in
reaching a somewhat higher latitude in sledges drawn by Esquimaux dogs.
But both expeditions agree in showing that the shores of Greenland
trend off suddenly towards the east at a point within some nine degrees
of the North Pole. On the other hand, the prolongation of the opposite
shore of Kennedy Channel was found to extend northwards as far as the
eye could reach. Within the angle thus formed there was an open sea
‘rolling,’ says Captain Maury, ‘with the swell of a boundless ocean.’
But a circumstance was noticed respecting this sea which was very
significant. The tides ebbed and flowed in it. Only one fact we know
of—a fact to be presently discussed—throws so much light on the
question we are considering as this circumstance does. Let us consider
a little whence these tidal waves can have come.
The narrow straits between Greenland on the one side, and Ellesmere
Land and Grinnell Land on the other, are completely ice-bound. We
cannot suppose that the tidal wave could have found its way beneath
such a barrier as this. ‘I apprehend,’ says Captain Maury, ‘that the
tidal wave from the Atlantic can no more pass under this icy barrier,
to be propagated in the seas beyond, than the vibrations of a musical
string can pass with its notes a fret on which the musician has placed
his finger.’
Are we to suppose, then, that the tidal waves were formed in the very
sea in which they were seen by Kane and Hayes? This is Captain Maury’s
opinion:—‘These tides,’ says he, ‘must have been born in that cold sea,
having their cradle about the North Pole.’
But if we carefully consider the theory of the tides, this opinion
seems inadmissible. Every consideration on which that theory is founded
is opposed to the assumption that the moon could by any possibility
raise tides in an arctic basin of limited extent. It would be out of
place to examine at length the principle on which the formation of
tides depends. It will be sufficient for our purposes to remark that it
is not to the mere strength of the moon’s ‘pull’ upon the waters of
any ocean that the tidal wave owes its origin, but to the difference of
the forces by which the various parts of that ocean are attracted. The
whole of an ocean cannot be raised at once by the moon; but if one part
is attracted more than another, a wave is formed. That this may happen,
the ocean must be one of wide extent. In the vast seas which surround
the Southern Pole there is room for an immensely powerful ‘drag,’ so to
speak; for always there will be one part of these seas much nearer to
the moon than the rest, and so there will be an appreciable difference
of pull upon that part.
The reader will now see why I have been so careful to ascertain the
limits of the supposed north-polar ocean, in which, according to
Captain Maury, tidal waves are generated. To accord with his views,
this ocean must be surrounded on all sides by impassable barriers
either of land or ice. These barriers, then, must lie to the northward
of the regions yet explored, for there is open sea communicating with
the Pacific all round the north of Asia and America. It only requires a
moment’s inspection of a terrestrial globe to see how small a space is
thus left for Captain Maury’s land-locked ocean. I have purposely left
out of consideration, as yet, the advances made by arctic voyagers in
the direction of the sea which lies between Greenland and Spitzbergen.
We shall presently see that on this side the imaginary land-locked
ocean must be more limited than towards the shores of Asia or America.
As it is, however, it remains clear, that if there were any ocean
communicating with the spot reached by Dr. Kane, but separated from
all communication—by open water—either with the Atlantic or with the
Pacific, that ocean would be so limited in extent that the moon’s
attraction could exert no more effective influence upon its waters
than upon the waters of the Mediterranean—where, as we know, no tides
are generated. This, then, would be a tideless ocean, and we must look
elsewhere for an explanation of the tidal waves seen by Dr. Kane.
We thus seem to have _primâ facie_ evidence that the sea reached
by Kane communicates either with the Pacific or with the Atlantic,
or—which is the most probable view—with both those oceans. When we
consider the voyages which have been made towards the North Pole along
the northerly prolongation of the Atlantic Ocean, we find very strong
evidence in favour of the view that there is open-water communication
in this direction, not only with the spot reached by Kane, but with a
region very much nearer to the North Pole.
So far back as 1607, Hudson had penetrated within eight and a half
degrees (or about 600 miles) of the North Pole on this route. When
we consider the clumsy build and the poor sailing qualities of the
ships of Hudson’s day, we cannot but feel that so successful a journey
marks this route as one of the most promising ever tried. Hudson was
not turned back by impassable barriers of land or ice, but by the
serious dangers to which the floating masses of ice and the gradually
thickening ice-fields exposed his weak and ill-manned vessel. Since
his time, others have sailed upon the same track, and hitherto with no
better success. It was reserved to the Swedish expedition of 1868 to
gain the highest latitudes ever reached in a ship in this direction.
The steamship ‘Sofia,’ in which this successful voyage was made, was
strongly built of Swedish iron, and originally intended for winter
voyages in the Baltic. Owing to a number of delays, it was not until
September 16 that the ‘Sofia’ reached the most northerly part of her
journey. This was a point some fifteen miles nearer the North Pole than
Hudson had reached. To the north there still lay broken ice, but packed
so thickly that not even a boat could pass through it. So late in the
season, it would have been unsafe to wait for a change of weather and
a consequent breaking-up of the ice. Already the temperature had sunk
sixteen degrees below the freezing-point; and the enterprising voyagers
had no choice but to return. They made, indeed, another push for the
north a fortnight later, but only to meet with a fresh repulse. An
ice-block with which they came into collision opened a large leak in
the vessel’s side; and when after great exertions they reached the
land, the water already stood two feet over the cabin floor. In the
course of these attempts, the depths of the Atlantic were sounded, and
two interesting facts were revealed. The first was that the island
of Spitzbergen is connected with Scandinavia by a submarine bank; the
second was the circumstance that to the north and west of Spitzbergen
the Atlantic is more than two miles deep!
We come now to the most conclusive evidence yet afforded of the
extension of the Atlantic Ocean towards the immediate neighbourhood
of the North Pole. Singularly enough, this evidence is associated not
with a sea-voyage, nor with a voyage across ice to the borders of
some northern sea, but with a journey during which the voyagers were
throughout surrounded as far as the eye could reach by apparently fixed
ice-fields.
In 1827 Sir Edward Parry was commissioned by the English Government to
attempt to reach the North Pole. A large reward was promised in case
he succeeded, or even if he could get within five degrees of the North
Pole. The plan which he adopted seemed promising. Starting from a port
in Spitzbergen, he proposed to travel as far northward as possible in
sea-boats, and then, landing upon the ice, to prosecute his voyage by
means of sledges. Few narratives of arctic travel are more interesting
than that which Parry has left of this famous ‘boat-and-sledge’
expedition. The voyagers were terribly harassed by the difficulties of
the way; and after a time, that most trying of all arctic experiences,
the bitterly cold wind which comes from out the dreadful north, was
added to their trials. Yet still they plodded steadily onwards,
tracking their way over hundreds of miles of ice with the confident
expectation of at least attaining to the eighty-fifth parallel, if not
to the Pole itself.
But a most grievous disappointment was in store for them. Parry began
to notice that the astronomical observations, by which in favourable
weather he estimated the amount of their northerly progress, showed
a want of correspondence with the actual rate at which they were
travelling. At first he could hardly believe that there was not some
mistake; but at length the unpleasant conviction was forced upon him
that the whole ice-field over which he and his companions had been
toiling so painfully was setting steadily southwards before the wind.
Each day the extent of this set became greater and greater, until at
length they were actually carried as fast towards the south as they
could travel northwards.
Parry deemed it useless to continue the struggle. There were certainly
two chances in his favour. It was possible that the north wind might
cease to blow, and it was also possible that the limit of the ice might
soon be reached, and that his boats might travel easily northward upon
the open sea beyond. But he had to consider the exhausted state of his
men, and the great additional danger to which they were subjected by
the movable nature of the ice-fields. If the ice should break up, or if
heavy and long-continued southerly winds should blow, they might have
found it very difficult to regain their port of refuge in Spitzbergen
before winter set in or their stores were exhausted. Besides, there
were no signs of water in the direction they had been taking. The
water-sky of arctic regions can be recognised by the experienced
seamen long before the open sea itself is visible. On every side,
however, there were the signs of widely-extended ice-fields. It seemed,
therefore, hopeless to persevere, and Parry decided on returning with
all possible speed to the haven of refuge prepared for the party
in Spitzbergen. He had succeeded in reaching the highest northern
latitudes ever yet attained by man. (A somewhat higher latitude has
since been reached by Captain Nares’s expedition.)
The most remarkable feature of this expedition, however, is not the
high latitude which the party attained, but the strange circumstance
which led to their discomfiture. What opinion are we to form of an
ocean at once wide and deep enough to float an ice-field which must
have been thirty or forty thousand square miles in extent? Parry had
travelled upwards of three hundred miles across the field, and we
may fairly suppose that he might have travelled forty or fifty miles
farther without reaching open water; also that the field extended fully
fifty miles on each side of Parry’s northerly track. That the whole of
so enormous a field should have floated freely before the arctic winds
is indeed an astonishing circumstance. On every side of this floating
ice-island there must have been seas comparatively free from ice;
and could a stout ship have forced its way through these seas, the
latitudes to which it could have reached would have been far higher
than those to which Parry’s party was able to attain. For a moment’s
consideration will show that the part of the great ice-field where
Parry was compelled to turn back must have been floating in far higher
latitudes when he first set out. He reckoned that he had lost more than
a hundred miles through the southerly motion of the ice-field, and by
this amount, of course, the point he reached had been nearer the Pole.
It is not assuming too much to say that a ship which could have forced
its way round the great floating ice-field would certainly have been
able to get within four degrees of the Pole. It seems to us highly
probable that she would even have been able to sail upon open water to
and beyond the Pole itself.
And when we remember the direction in which Dr. Kane saw an open
sea—namely, towards the very region where Parry’s ice—ship had floated
a quarter of a century before—it seems reasonable to conclude that
there is open water communication between the seas which lie to the
north of Spitzbergen and those which lave the north-western shores of
Greenland. If this be so, we at once obtain an explanation of the tidal
waves which Kane watched day after day in 1855. These had no doubt
swept along the valley of the Atlantic, and thence around the northern
coast of Greenland. It follows that, densely as the ice may be packed
at times in the seas by which Hudson, Scoresby, and other captains have
attempted to reach the North Pole, the frozen masses must in reality
be floating freely, and there must therefore exist channels through
which an adventurous seaman might manage to penetrate the dangerous
barriers surrounding the polar ocean.
In such an expedition, chance unfortunately plays a large part. Whalers
tell us that there is great uncertainty as to the winds which may
blow during an arctic summer. The icebergs may be crowded by easterly
winds upon the shores of Greenland, or by westerly winds upon the
shores of Spitzbergen, or, lastly, the central passage may be the most
encumbered, through the effects of winds blowing now from the east and
now from the west. Thus the arctic voyager has not merely to take his
chance as to the route along which he shall adventure northwards, but
often, after forcing his way successfully for a considerable distance,
he finds the ice-fields suddenly closing in upon him on every side,
and threatening to crush his ship into fragments. The irresistible
power with which, under such circumstances, the masses of ice bear
down upon the stoutest ship, has been evidenced again and again;
though, fortunately, it not unfrequently happens that some irregularity
along one side or the other of the closing channel serves as a sort
of natural dock, within which the vessel may remain in comparative
safety until a change of wind sets her free. Instances have been known
in which a ship has had so narrow an escape in this way, and has
been subjected to such an enormous pressure, that when the channel
was opened out again, the impress of the ship’s side has been seen
distinctly marked upon the massive blocks of ice which have pressed
against her.
(From the _St. Paul’s Magazine_, June 1869.)
FOOTNOTES:
[5] So far back as 1789, John Williams, in his _Natural History of
the Mineral Kingdom_, discussed the question of the ‘Limited Quantity
of Coal in Great Britain.’ The following extracts are taken from an
excellent paper on the exhaustion of our coal in the _Popular Science
Review_ for July 1866, by Mr. Lemoran, Colliery Viewer. ‘I have no
doubt,’ says Williams, ‘that the generality of the inhabitants of
Great Britain believe that our coal mines are inexhaustible; and the
general conduct of the nation, so far as relates to this subject,
seems to imply that this is held as an established fact. If it was
not a generally received opinion, would the rage for exporting coals
be allowed to go on without limitation or remorse? But it is full
time that the public were undeceived in a matter which so nearly
concerns the welfare of this flourishing island.... When our coal
mines are exhausted, the prosperity and glory of this flourishing and
fortunate island are at an end. Our cities and great towns must then
become ruinous heaps for want of fuel, and our mines and manufactories
must fail from the same cause, and then, consequently, our commerce
must vanish. In short, the commerce, wealth, importance, glory, and
happiness of Great Britain will decay and gradually dwindle away to
nothing, in proportion as our coal and other mines fail.’ Mr. Williams
also solves in a very summary manner the problem of England’s fate
after her coal stores shall be exhausted. ‘The future inhabitants
of this island must live,’ says he, ‘like its first inhabitants, by
fishing and hunting.’
[6] In 1854, the yield was 64,661,401 tons; in 1864, the yield was
92,787,873: the average increase per annum was, therefore, no less than
2,812,647 tons.
[7] I have obtained a somewhat different result from a computation
I have just gone through. I make the consumption 291 millions in
1900, and 1,446 millions in 1950. Mr. Lemoran seems to have taken
the percentage at 3½ instead of 3¼. It is worth noticing how
seriously a small change in the percentage affects the result; the
consumption in 1950 becoming 1,760 millions of tons, instead of 1,446
millions.
[8] The year 1863 was the last whose statistics were available for Mr.
Jevons’s purpose; and estimating from either 1860 or 1862 would give a
result smaller than either of the above. Indeed, the consumption was
less in 1862 than in 1861.
_IS THE GULF STREAM A MYTH?_
The Gulf Stream has recently attracted a large share of the attention
of our men of science. The strange weather which we experienced
last winter (see date of essay) has had something to do with this.
The influence of the Gulf Stream upon our climate, and the special
influence which it is assumed to exercise in mitigating the severity
of our winters, have been so long recognised that meteorologists began
to inquire what changes could be supposed to have taken place in the
great current to account for so remarkable a winter as the last. But
it happened also that at a meeting of the Royal Geographical Society
early in the present year the very existence of the Gulf Stream was
called in question, just when meteorologists were disposed to assign to
it effects of unusual importance. And in the course of the discussion
whether there is in truth a Gulf Stream—or rather whether our shores
are visited by a current which merits such a name—a variety of
interesting facts were adduced, which were either before unknown or had
attracted little attention. As at a recent meeting of the same society
these doubts have been renewed, I propose to examine briefly, in the
first place, a few of the considerations which have been urged against
the existence of a current from the Gulf of Mexico to the neighbourhood
of our shores; and then, having rehabilitated the reputation of
this celebrated ocean river—as I believe I shall be able to do—I
shall proceed to give a brief sketch of the processes by which the
current-system of the North Atlantic is set and maintained in motion.
In reality the Gulf Stream is only a part of a system of oceanic
circulation; but in dealing with the arguments which have been urged
against its very existence, we may confine our attention to the fact
that, according to the views which had been accepted for more than a
century, there is a stream of water which, running out of the Gulf
Stream through the Narrows of Bemini, flows along the shores of the
United States to Newfoundland, and thence right across the Atlantic to
the shores of Great Britain. It is this last fact which is now called
in question. The existence of a current as far as the neighbourhood of
Newfoundland is conceded, but the fact that the stream flows onward to
our shores is denied.
The point on which most stress is placed is the shallowness of the
passage called the ‘Bemini Narrows,’ through which it is assumed that
the whole of the Gulf current must pass. This passage has a width of
about forty miles, and a depth of little more than six hundred yards.
The current which flows through it is perhaps little more than thirty
miles in width, and a quarter of a mile in depth. It is asked with
some appearance of reason, how this narrow current can be looked upon
as the parent of that wide stream which is supposed to traverse the
Atlantic with a mean width of some five or six hundred miles. Indeed,
a much greater width has been assigned to it, though on mistaken
grounds; for it has been remarked that since waifs and strays from the
tropics are found upon the shores of Portugal, as well as upon those of
Greenland, we must ascribe to the current a span equal to the enormous
space separating these places. But the circumstance here dwelt upon can
clearly be explained in another way. We know that of two pieces of wood
thrown into the Thames at Richmond, one might be picked up at Putney,
and the other at Gravesend. Yet we do not conclude that the width of
the Thames is equal to the distance separating Putney from Gravesend.
And doubtless the tropical waifs which have been picked up on the
shores of Greenland and of Portugal have found their way thither by
circuitous courses, and not by direct transmission along opposite edges
of the great Gulf current.
But certainly the difficulty associated with the narrowness of the
Bemini current is one deserving of careful attention. Are we free to
identify a current six hundred miles in width with one which is but
thirty miles wide, and not very deep? An increase of width certainly
not less than thirtyfold would appear to correspond to a proportionate
diminution of depth. And remembering that it is only near the middle of
the Narrows that the Gulf Stream has a depth of four hundred yards, we
could scarcely assign to the wide current in the mid-Atlantic a greater
depth than ten or twelve yards. This depth seems altogether out of
proportion to the enormous lateral extension of the current.
But besides that even this consideration would not suffice to
disprove the existence of a current in the mid-Atlantic, an important
circumstance remains to be mentioned. The current in the Narrows flows
with great velocity,—certainly not less than four or five miles an
hour. As the current grows wider it flows more sedately; and opposite
Cape Hatteras its velocity is already reduced to little more than
three miles an hour. In the mid-Atlantic the current may be assumed to
flow at a rate little exceeding a mile per hour, at the outside. Here,
then, we have a circumstance which suffices to remove a large part of
the difficulty arising from the narrowness of the Bemini current, and
we can at once increase our estimate of the depth of the mid-Atlantic
current fivefold.
But this is not all. It has long been understood that the current
which passes out through the Narrows of Bemini corresponds to the
portion of the great equatorial current which passes into the Gulf
of Mexico between the West Indian Islands. We cannot doubt that the
barrier formed by those islands serves to divert a large portion of
the equatorial current. The portion thus diverted finds its way, we
may assume, along the outside of the West Indian Archipelago, and thus
joins the other portion—which has in the meantime made the circuit of
the Gulf—as it issues from the Bemini Straits. All the maps in which
the Atlantic currents are depicted present precisely such an outside
current as I have here spoken of, and most of them assign to it a width
exceeding that of the Bemini current. Indeed, were it not for the
doubts which the recent discussions have thrown upon all the currents
charted by seamen, I should have been content to point to this outside
current as shown in the maps. As it is, I have thought is necessary
to show that such a current must necessarily have an existence, since
we cannot lose sight of the influence of the West Indian Isles in
partially damming up the passage along which the equatorial current
would otherwise find its way into the Gulf of Mexico. Whatever portion
of the great current is thus diverted must find a passage elsewhere,
and no passage exists for it save along the outside of the West Indian
Isles.
The possibility that the wide current which has been assumed to
traverse the mid-Atlantic may be associated with the waters which
flow from the Gulf of Mexico, either through the Narrows or round
the outside of the barrier formed by the West Indies, has thus been
satisfactorily established. But we now have to consider difficulties
which have been supposed to encounter our current on its passage from
the Gulf to the mid-Atlantic.
Northwards, along the shores of the United States, the current has been
traced by the singular blueness of its waters until it has reached the
neighbourhood of Newfoundland. Over a part of this course, indeed,
the waters of the current are of indigo blue, and so clearly marked
that their line of junction with the ordinary sea-water can be traced
by the eye. ‘Often,’ says Captain Maury, ‘one half of a vessel may
be perceived floating in Gulf Stream water, while the other half is
in common water of the sea—so sharp is the line, and such the want
of affinity between the waters, and such, too, the reluctance, so to
speak, on the part of those of the Gulf Stream, to mingle with the
littoral waters of the sea.’
But it is now denied that there is any current beyond the neighbourhood
of Newfoundland—or that the warm temperature, which has characterised
the waters of the current up to this point, can be detected farther out.
It is first noticed that, as the Gulf current must reach the
neighbourhood of Newfoundland with a north-easterly motion, and, if
it ever reached the shores of the British Isles, would have to travel
thither with an almost due easterly motion, there is a change of
direction to be accounted for. This, however, is an old, and I had
supposed exploded, fallacy. The course of the Gulf Stream from the
Bemini Straits to the British Isles corresponds exactly with that which
is due to the combined effects of the motion of the water and that of
the earth upon its axis. Florida being much nearer than Ireland to
the equator, has a much more rapid easterly motion. Therefore, as the
current gets farther and farther north, the effect of the easterly
motion thus imparted to it begins to show itself more and more, until
the current is gradually changed from a north-easterly to an almost
easterly stream. The process is the exact converse of that by which the
air-currents from the north gradually change into the north-westerly
trade-winds as they get farther south.
But it is further remarked that as the current passes out beyond the
shelter of Newfoundland, it is impinged upon by those cold currents
from the arctic seas which are known to be continually flowing out
of Baffin’s Bay and down the eastern shores of Greenland; and it is
contended that these currents suffice, not merely to break up the
Gulf current, but so to cool its waters that these could produce no
effect upon the climate of Great Britain if they ever reached its
neighbourhood.
Here, again, I must remark that we are dealing with no new discovery.
Captain Maury has already remarked upon this peculiarity. ‘At the very
season of the year,’ he says, ‘when the Gulf Stream is rushing in
greatest volume through the Straits of Florida, and hastening to the
north with the greatest rapidity, there is a cold stream from Baffin’s
Bay, Labrador, and the coasts of the north, running south with equal
velocity.... One part of it underruns the Gulf Stream, as is shown
by the icebergs, which are carried in a direction tending across its
course.’ There can be no doubt, in fact, that this last circumstance
indicates the manner in which the main contest between the two currents
is settled. A portion of the arctic current finds its way between the
Gulf Stream and the continent of America; and this portion, though
narrow, has a very remarkable effect in increasing the coldness of
the American winters. But the main part, (heavier, by reason of its
coldness, than the surrounding water,) sinks beneath the surface. And
the well-known fact mentioned by Maury, that icebergs have been seen
stemming the Gulf Stream, suffices to show how comparatively shallow
that current is at this distance from its source, and thus aids to
remove a difficulty which we have already had occasion to deal with.
Doubtless the cooling influence of the arctic currents is appreciable;
but it would be a mistake to suppose that this influence can suffice
to deprive the Gulf current of its distinctive warmth. If all the
effect of the cold current were operative on the Gulf Stream alone we
might suppose that, despite the enormous quantity of comparatively
warm water which is continually being carried northwards, the current
would be reduced to the temperature of the surrounding water. But
this is not so. The arctic current not only cools the Gulf current,
but the surrounding water also—possibly to a greater extent, for it
is commonly supposed that a bed of ordinary sea-water separates the
two main currents from each other. Thus the characteristic difference
of temperature remains unaffected. But in reality we may assume that
the cooling effect actually exercised by the arctic current upon the
neighbouring sea is altogether disproportionate to the immense amount
of heat continually being carried northwards by the Gulf Stream.
It is astonishing how unreadily two sea-currents exchange their
temperatures—to use a somewhat inexact mode of expression. The very
fact that the littoral current of the United States is so cold—a fact
thoroughly established—shows how little warmth this current has drawn
from the neighbouring seas. Another fact, mentioned by Captain Maury,
bears in a very interesting manner upon this peculiarity. He says:
‘If any vessel will take up her position a little to the northward of
Bermuda, and steering thence for the capes of Virginia, will try the
water-thermometer all the way at short intervals, she will find its
reading to be now higher, now lower; and the observer will discover
that he has been crossing streak after streak of warm and cool water in
regular alternations.’ Each portion maintains its own temperature, even
in the case of such warm streaks as these, all belonging to one current.
Similar considerations dispose of the arguments which have been founded
on the temperature of the sea-bottom. It has been proved that the
living creatures which people the lower depths of the sea exist under
circumstances which evidence a perfect uniformity of temperature; and
arguments on the subject of the Gulf Stream have been derived from the
evidence of what is termed a minimum thermometer—that is, a thermometer
which will indicate the lowest temperature it has been exposed to—let
down into the depths of the sea. All such arguments, whether adduced
against or in favour of the Gulf Stream theory, must be held, to be
futile, since the thermometer in its descent may pass through several
submarine currents of different temperature.
Lastly, an argument has been urged against the warming effects of the
Gulf Stream upon our climate which requires to be considered with
some attention. It is urged that the warmth derived from so shallow
a current as the Gulf Stream must be, by the time it has reached our
shores, could not provide an amount of heat sufficient to affect our
climate to any appreciable extent. The mere neighbourhood of this water
at a temperature slightly higher than that due to the latitude could
not, it is urged, affect the temperature of the inland counties at all.
This argument is founded on a misapprehension of the beautiful
arrangement by which Nature carries heat from one region to distribute
it over another. Over the surface of the whole current the process of
evaporation is going on at a greater rate than over the neighbouring
seas, because the waters of the current are warmer than those which
surround them. The vapour thus rising above the Gulf Stream is
presently wafted by the south-westerly winds to our shores and over
our whole land. But as it thus reaches a region of comparative cold,
the vapour is condensed—that is, turned into fog, or mist, or cloud,
according to circumstances. It is during this change that it gives out
the heat it has brought with it from the Gulf Stream. For precisely as
the evaporation of water is a process requiring heat, the change of
vapour into water—whether in the form of fog, mist, cloud, or rain—is a
process in which heat is given out. Thus it is that the south-westerly
wind, the commonest wind we have, brings clouds and fogs and rain to us
from the Gulf Stream, and with them brings the Gulf Stream warmth.
Why the south-westerly winds should be so common, and how it is that
over the Gulf Stream there is a sort of air-channel along which winds
come to us as if by their natural pathway, are matters inquired into
farther on (see p. 164). The subject is full of interest, but need not
here detain us.
It would seem that a mechanism involving the motion of such enormous
masses of water as the current-system of the Atlantic should depend
on the operation of very evident laws. Yet a variety of contradictory
hypotheses have been put forward from time to time respecting this
system of circulation, and even now the scientific world is divided
between two opposing theories.
Of old the Mississippi River was supposed to be the parent of the Gulf
Stream. It was noticed that the current flows at about the same rate as
the Mississippi, and this fact was considered sufficient to support the
strange theory that a river can give birth to an ocean-current.
It was easy, however, to overthrow this theory. Captain Livingston
showed that the volume of water which is poured out of the Gulf of
Mexico in the form of an ocean stream is more than a thousand times
greater than the volume poured into the Gulf by the Mississippi River.
Having overthrown this old theory of the Gulf Stream, Captain
Livingston attempted to set up one which is equally unfounded. He
ascribed the current to the sun’s apparent yearly motion and the
influence thus exerted on the waters of the Atlantic. A sort of yearly
tide is conceived, according to this theory, to be the true parent of
the Gulf current. It need hardly be said, however, that a phenomenon
which remains without change through the winter and summer seasons
cannot possibly be referred to the operation of such a cause as a
yearly tide.
It is to Dr. Franklin that we owe the first theory of the Gulf Stream
which has met with general acceptance. He held that the Gulf Stream
is formed by the outflow of waters which have been forced into the
Caribbean Sea by the trade-winds; so that the pressure of these winds
on the Atlantic Ocean forms, according to Dr. Franklin, the true motive
power of the Gulf Stream machinery. According to Maury, this theory
has ‘come to be the most generally received opinion in the mind of
seafaring people.’ It supplies a moving force of undoubted efficiency.
We know that as the trade-winds travel towards the equator they lose
their westerly motion. It is reasonable to suppose that this is caused
by friction against the surface of the ocean, to which, therefore, a
corresponding westerly motion must have been imparted.
There is a simplicity about Franklin’s theory which commends it
favourably to consideration. But when we examine it somewhat more
closely, several very decided flaws present themselves to our attention.
Consider, in the first place, the enormous mass of water moved by the
supposed agency of the winds. Air has a weight—volume for volume—which
is less than one eight-hundredth part of that of water. So that, to
create a water-current, an air-current more than eight hundred times as
large and of equal velocity must expend the whole of its motion. Now
the trade-winds are gentle winds, their velocity scarcely exceeding in
general that of the more swiftly-moving portions of the Gulf Stream.
But even assigning to them a velocity four times as great, we still
want an air-current two hundred times as large as the water-current.
And the former must give up the whole of its motion, which, in the case
of so elastic a substance as air, would hardly happen, the upper air
being unlikely to be much affected by the motion of the lower.
But this is far from being all. If the trade-winds blew throughout
the year, we might be disposed to recognise their influence upon the
Gulf Stream as a paramount, if not the sole one. But this is not the
case. Captain Maury states that, ‘With the view of ascertaining the
average number of days during the year that the north-east trade-winds
of the Atlantic operate upon the currents between twenty-five degrees
north latitude and the equator, log-books containing no less than
380,284 observations on the force and direction of the wind in that
ocean were examined. The data thus afforded were carefully compared
and discussed. The results show that within these latitudes—and on the
average—the wind from the north-east is in excess of the winds from
the south-west only 111 days out of the 365. Now, can the north-east
trades,‘ he pertinently asks, ‘by blowing for less than one-third
of the time, cause the Gulf Stream to run all the time, and without
varying its velocity either to their force or to their prevalence?’
And besides this, we have to consider that no part of the Gulf Stream
flows strictly before the trade-winds. Where the current flows most
rapidly, namely, in the Narrows of Bemini, it sets against the wind,
and for hundreds of miles after it enters the Atlantic ‘it runs,’ says
Maury, ‘right in the “wind’s eye.”‘ It must be remembered that a
current of air directed with considerable force against the surface of
still water has not the power of generating a current which can force
its way far through the resisting fluid. If this were so, we might
understand how the current, originating in sub-tropical regions, could
force its way onward after the moving force had ceased to act upon it,
and even carry its waters right against the wind, after leaving the
Gulf of Mexico. But experience is wholly opposed to this view. The
most energetic currents are quickly dispersed when they reach a wide
expanse of still water. For example, the Niagara below the falls is an
immense and rapid river. Yet when it reaches Lake Ontario, ‘instead
of preserving its character as a distinct and well-defined stream
for several hundred miles, it spreads itself out, and its waters are
immediately lost in those of the lake.’ Here, again, the question asked
by Maury bears pertinently on the subject we are considering. ‘Why,’ he
says, ‘should not the Gulf Stream do the same? It gradually enlarges
itself, it is true; but, instead of mingling with the ocean by broad
spreading, as the immense rivers descending into the northern lakes do,
its waters, like a stream of oil in the ocean, preserve a distinctive
character for more than three thousand miles.’
The only other theory which has been considered in recent times
to account satisfactorily for all the features of the Gulf Stream
mechanism was put forward, we believe, by Captain Maury. In this
theory, the motive power of the whole system of oceanic circulation is
held to be the action of the sun’s heat upon the waters of the sea. We
recognise two contrary effects as the immediate results of the sun’s
action. In the first place, by warming the equatorial waters, it tends
to make them lighter; in the second place, by causing evaporation, it
renders them salter, and so tends to make them heavier. We have to
inquire which form of action is most effective. The inquiry would be
somewhat difficult, if we had not the evidence of the sea itself to
supply an answer. For it is an inquiry to which ordinary experimental
processes would not be applicable. We must accept the fact that the
heated water from the equatorial seas actually does float upon the
cooler portions of the Atlantic, as evidence that the action of the
sun results in making the water lighter.
Now, Maury says that the water thus lightened must flow over and form
a surface-current towards the Poles; while the cold and heavy water
from the polar seas, as soon as it reaches the temperate zone, must
sink and form a submarine current. He recognises in these facts the
mainspring of the whole system of oceanic circulation. If a long trough
be divided into two compartments, and we fill one with oil and the
other with water, and then remove the dividing plate, we shall see the
oil rushing over the water at one end of the trough, and the water
rushing under the oil at the other. And if we further conceive that oil
is continually being added at that end of the trough originally filled
with oil, while water is continually added at the other, it is clear
that the system of currents would continue in action: that is, there
would be a continual flow of oil in one direction along the surface of
the water, and of water in the contrary direction underneath the oil.
But Sir John Herschel maintains that no such effects as Maury describes
could follow the action of the sun’s heat upon the equatorial waters.
He argues thus: Granting that these waters become lighter and expand in
volume, yet they can only move upwards, downwards, or sideways. There
can be nothing to cause either of the two first forms of motion; and as
for motion sideways, it can only result from the gradual slope caused
by the bulging of the equatorial waters. He proceeds to show that this
slope is so slight that we cannot look upon it as competent to form any
sensible current from the equatorial towards the polar seas. And even
if it could, he says, the water thus flowing off would have an eastward
instead of a westward motion, precisely as the counter-trade-winds,
blowing from equatorial to polar regions, have an eastward motion.
It is singular how completely the supporter of each rival view has
succeeded in overthrowing the arguments of his opponent. Certainly
Maury has shown with complete success that the inconstant trade-winds
cannot account for the constant Gulf current, which does not even flow
before them, but, in places, exactly against their force. And the
reasoning of Sir John Herschel seems equally cogent, for certainly the
flow of water from equatorial towards polar regions ought from the
first to have an eastward, instead of a westward motion; whereas the
equatorial current, of which the Gulf Stream is but the continuation,
flows from east to west, right across the Atlantic.
Equally strange is it to find that each of these eminent men, having
read the arguments of the other, reasserts, but does not effectually
defend, his own theory, and repeats with even more damaging effect his
arguments against the rival view.
Yet one or other theory must at least point to the true view, for the
Atlantic is subject to no other agencies which can for a moment be held
to account for a phenomenon of such magnitude as the Gulf Stream.
It appears to me, that on a close examination of the Gulf Stream
mechanism, the true mainspring of its motion can be recognised.
Compelled to reject the theory that the trade-winds generate the
equatorial current westward, let us consider whether Herschel’s
arguments against the ‘heat theory’ may not suggest a hint for our
guidance. He points out that an overflow from the equator polewards
would result in an eastward, and not in a westward, current. This is
true. It is equally true that a flow of water towards the equator
would result in a westward current. But no such flow is observed. Is
it possible that there may be such a flow, but that it takes place in
a hidden manner? Clearly there may be. Submarine currents towards the
equator would have precisely the kind of motion we require, and if any
cause drew them to the surface near the equator, they would account in
full for the great equatorial westward current.
At this point we begin to see that an important circumstance has been
lost sight of in dealing with the heat theory. The action of the sun on
the surface-water of the equatorial Atlantic has only been considered
with reference to its warming effects. But we must not forget that
this action has drying effects also. It evaporates enormous quantities
of water, and we have to inquire whence the water comes by which the
sea-level is maintained. A surface flow from the sub-tropical seas
would suffice for this purpose, but no such flow is observed. Whence,
then, can the water come but from below? Thus we recognise the fact
that a process resembling suction is continually taking place over
the whole area of the equatorial Atlantic, the agent being the intense
heat of the tropical sun. No one can doubt that this agent is one of
adequate power. Indeed, the winds, conceived by Franklin to be the
primary cause of the Atlantic currents, are in reality due to the
merest fraction of the energy inherent in the sun’s heat.
We have other evidence that the indraught is from below in the
comparative coldness of the equatorial current. The Gulf Stream is warm
by comparison with the surrounding waters, but the equatorial current
is cooler than the tropical seas. According to Professor Ansted, the
southern portion of the equatorial current, as it flows past Brazil,
‘is everywhere a cold current, generally from four to six degrees below
the adjacent ocean.’
If we here recognise the mainspring of the Gulf Stream mechanism, or
rather of the whole system of oceanic circulation-for the movements
observed in the Atlantic have their exact counterpart in the Pacific—we
shall have no difficulty in accounting for all the motions which that
mechanism exhibits. We need no longer look upon the Gulf Stream as the
rebound of the equatorial current from the shores of North America.
Knowing that there is an underflow towards the equator, we see that
there must be a surface-flow towards the Poles. And this flow must as
inevitably result in an easterly motion as the underflow towards the
equator results in a westerly motion. We have, indeed, the phenomena
of the trades and counter-trades exhibited in water-currents instead of
air-currents.
(From the _St. Paul’s Magazine_, September 1869.)[9]
_FLOODS IN SWITZERLAND._
Recently (see date of essay) we have witnessed a succession of
remarkable evidences of Nature’s destructive powers. The fires of
Vesuvius, the earth-throes of the sub-equatorial Andes, and the
submarine disturbance which has shaken Hawaii, have presented to us the
various forms of destructive action which the earth’s, subterranean
forces can assume. In the disastrous floods which have recently visited
the Alpine cantons of Switzerland, we have evidence of the fact that
natural forces which we are in the habit of regarding as beneficent and
restorative may exhibit themselves as agents of the most widespread
destruction. I have pointed out elsewhere (see p. 226) how enormous
is the amount of power of which the rain-cloud is the representative;
and in doing so I have endeavoured to exhibit the contrast between the
steady action of the falling shower and the energy of the processes of
which rain is in reality the equivalent. But in the floods which have
lately ravaged Switzerland we see the same facts illustrated, not by
numerical calculations or by the results of philosophical experiments,
but in action, and that action taking place on the most widely extended
scale. The whole of the south-eastern, or, as it may be termed, the
Alpine half of Switzerland, has suffered from these floods. If a line
be drawn from the Lake of Constance, in the north-east of Switzerland,
to the Col de Balme, in the south-west, it will divide Switzerland into
two nearly equal portions, and scarcely a canton within the eastern of
these divisions has escaped without great damage.
The cantons which have suffered most terribly are those of Tessin,
Grisons, and St. Gall. The St. Gothard, Splugen, and St. Bernhardin
routes have been rendered impassable. Twenty-seven lives were lost in
the St. Gothard Pass, besides horses and waggons full of merchandise.
It is stated that on the three routes upwards of eighty persons
perished. In the village of Loderio alone, no less than fifty deaths
occurred. So terrible a flood has not taken place since the year 1834.
Nor have the cantons of Uri and Valais escaped. From Unterwalden we
hear that the heavy rains which took place a fortnight ago have carried
away several large bridges, and many of the rivers continue still very
swollen. I have already described how enormous the material losses are
which have been caused by these floods. Many places are under water;
others in ruins or absolutely destroyed. In Tessin alone the damage is
estimated at forty thousand pounds sterling.
A country like Switzerland must always be liable to the occurrence,
from time to time, of catastrophes of this sort. Or rather, perhaps,
we should draw a distinction between the two divisions of Switzerland
referred to above. Of these the one may be termed the mountain half,
and the other the lake half of the country. It is the former portion
of the country which is principally subject to the dynamical action of
water. A long-continued and heavy rainfall over the higher lands cannot
fail to produce a variety of remarkable effects, where the arrangement
of mountains and passes, hills, valleys, and ravines is so complicated.
There are places where a large volume of water can accumulate until the
barriers which have opposed its passage to the plains burst under its
increasing weight; and then follow those destructive rushes of water
which sweep away whole villages at once. It is, in fact, the capacity
of the Swiss mountain region for damming up water, far more than any
other circumstance, which renders the Swiss floods so destructive.
And then it must be remembered that there are at all times suspended
over the plains and valleys which lie beneath the Alpine ranges
enormous masses of water in the form of snow and ice. Although in
general these suffer no changes but those due to the partial melting
which takes place in summer, and the renewed accumulation which takes
place in winter, yet when heavy rains fall upon the less elevated
portions of the Alpine snow, they not only melt that snow much more
rapidly than the summer sun would do, but they wash down large masses,
which add largely to the destructive power of the descending waters.
The most destructive floods which have occurred in Switzerland have
usually been those which take place in early summer. The floods which
inundated the plains of Martigny in 1818 were a remarkable instance of
the effects which result from the natural damming up of large volumes
of water in the upper parts of the Alpine hill-country. The whole of
the valley of Bagnes, one of the largest of the lateral branches of
the main valley of the Rhone above Geneva, was converted into a lake,
in the spring of 1818, by the damming up of a narrow pass into which
avalanches of snow and ice had been precipitated from a lofty glacier
overhanging the bed of the river Dranse. The ice barrier enclosed a
lake no less than half a league in length and an eighth of a mile
wide, and in places two hundred feet deep. The inhabitants of the
neighbouring villages were terrified by the danger which was to be
apprehended from the bursting of the barrier. They cut a gallery seven
hundred feet long through the ice, while the waters had as yet risen
to but a moderate height; and when the waters began to flow through
this channel, its course was deepened by the melting of the ice, and
at length nearly half the contents of the lake were safely carried
off. It was hoped that the process would continue, and the country be
saved from the danger which had been so long impending over it. But
as the heat of the weather increased, the central part of the barrier
slowly melted away, until it became too weak to bear the enormous
weight of water which was pressing against it. At length it gave way,
so suddenly and completely that all the water which remained in the
lake rushed out in half an hour. The downward passage of the water
illustrated, in a very remarkable way, the fact that the chief mischief
of floods is occasioned where water is checked in its outflow. For it
is related that, ‘in the course of their descent the waters encountered
several narrow gorges, and at each of these they rose to a great
height, and then burst with new violence into the next basin, sweeping
along forests, houses, bridges, and cultivated land.’ Along the greater
part of its course the flood resembled rather a moving mass of rock and
mud than a stream of water. Enormous masses of granite were torn out
of the sides of the valleys and whirled for hundreds of yards along
the course of the flood. M. Escher relates that one of the fragments
thus swept along was no less than sixty yards in circumference. At
first the water rushed onwards at a rate of more than a mile in three
minutes, and the whole distance (forty-five miles) which separates the
valley of Bagnes from the Lake of Geneva was traversed in little more
than six hours. The bodies of persons who had been drowned in Martigny
were found floating on the farther side of the lake of Geneva, near
Vevey. Thousands of trees were torn up by the roots, and the ruins of
buildings which had been overthrown by the flood were carried down
beyond Martigny. In fact, the flood at this point was so high that
some of the houses in Martigny were filled with mud up to the second
storey.‘ Beyond Martigny the flood did but little damage, as it here
expanded over the plain, and was reduced both in depth and velocity.
(From the _Daily News_ for October 20, 1868.)
FOOTNOTES:
[9] See ‘Light Science’ (second series) for a discussion of later
researches.
_A GREAT TIDAL WAVE._
During the last few days anxious questionings have been heard
respecting the next spring tides. A certain naval officer, who
conceives that he can trace in the relative positions of the sun and
moon the secret of every important change of weather, has described
in the columns of a contemporary the threatening significance of the
approaching conjunction of the sun and moon. He predicts violent
atmospheric disturbances; though in another place he tells us merely
that the conjunction is to cause ‘unsettled weather,’ a state of
matters to which we in England have become tolerably well accustomed.
But people are asking what is the actual relation which is to bring
about such terrible events. The matter is very simple. On October 5,
the moon will be new—in other words, if it were not for the brightness
of the sun, we should see the moon close by that luminary on the
heavens. Thus the sun and moon will pull with combined effect upon
the waters of the earth, and so cause what are called spring tides.
This, of course, happens at the time of every new moon, but sometimes
the moon exerts a more effective pull than at other times; and the
same happens also in the case of the sun; and on October 5, it happens
that both the sun and the moon will give a particularly vigorous
haul upon the earth’s waters. As regards the sun, there is nothing
unusual. Every October his pull on the ocean is much the same as in
preceding Octobers. But October is a month of high solar tides—and for
these reasons:—In September, as everyone knows, the sun crosses the
equinoctial; and, other things being equal, it would be when on the
equinoctial that his power to raise a tidal wave would be greatest.
But other things are not equal; for the sun is not always at the same
distance from the earth. He is nearest in January; so that he would
exert more power in that month than in any other, if his force depended
solely on distance. As matters actually stand, it will be obvious that
at some time between September and January the sun’s tidal power would
have a maximum value. Thus it is that October is a month of high solar
tidal waves.
But it is the lunar wave which will be most effectively strengthened
at the next spring tide. If we could watch the lunar tidal wave alone
(instead of always finding it combined with the solar wave) we should
find it gradually increasing, and then gradually diminishing, in a
period of about a lunar month. And we should find that it was always
largest when the moon looked largest, and _vice versâ_. In other words,
when the moon is in perigee the lunar wave is largest. But then there
is another consideration. The lunar wave would vary according to the
moon’s proximity to the equinoctial; and (other things being equal)
would be largest when the moon is exactly opposite the earth’s equator.
If the two effects are combined, that is, if the moon happens to be in
perigee and on the equinoctial at the same time, then of course we get
the largest lunar tidal wave we can possibly have.
Now this ‘largest lunar wave’ occurs at somewhat long intervals,
because the relation on which it depends is one which is, so to speak,
exceptional. Still the relation does recur, and with a certain degree
of regularity. When it happens, however, it by no means follows that
we have a very high tide; because it may occur when the tides are near
‘neap’; in other words, when the sun and moon exert opposing effects.
The largest lunar wave cannot stand the drain which the solar wave
exerts upon it at the time of neap tides. Nor would the large lunar
tidal wave produce an exceptionally high tide, even though it were
not the time of ‘neap,’ or were tolerably near the time of ‘spring’
tides. Only when it happens that a large lunar wave combines fully with
the solar wave, do we get very high tides. And when, in addition to
this relation, we have the solar wave nearly at a maximum, we get the
highest of all possible tides. This is what will happen, or all but
happen, on October 5 next. The combination of circumstances is almost
the most effective that can possibly exist.
But, after all, high tides depend very importantly on other
considerations than astronomical ones. Most of us remember how
a predicted high tide some two years ago turned out to be a very
moderate, or, if we may use the expression, a very ‘one-horse’ affair
indeed, because the winds had not been consulted, and exerted their
influence against the astronomers. A long succession of winds blowing
off-shore would reduce a spring tide to a height scarcely exceeding the
ordinary neap. On the other hand, if we should have a long succession
of westerly winds from the Atlantic before the approaching high tide,
it is certain that a large amount of mischief may be done in some of
our riverside regions.[10]
As for the predicted weather changes, they may be regarded as mere
moonshine. A number of predictions, founded on the motions of the sun
and moon, have found a place during many months past in the columns of
a contemporary; but there has been no greater agreement between these
predictions and the weather actually experienced than anyone could
trace between Old Moore’s weather prophecies and recorded weather
changes. In other words, there have been certain accordances which
would be very remarkable indeed if they did not happen to be associated
with as many equally remarkable discordances. Random predictions would
be quite as satisfactory.
A very amusing misprint has found its way into many newspapers in
connection with the coming tide. It is interesting as serving to show
how little is really known by the general public about some of the
simplest scientific matters. The original statement announced that the
sun would not be in perihelion by so many seconds of semi-diameter, in
itself a very incorrect mode of expression. Still it was clear that
what was meant was, that the earth would be so far from the place of
nearest approach to the sun that the latter would not look as large
as it possibly can look, by so many seconds of semi-diameter. In many
papers, however, we read that the ‘sun will not be in perihelion by so
many seconds of mean chronometer!’ Who first devised this marvellous
reading is unknown.
(From the _Daily News_ for September 27, 1869.)
FOOTNOTES:
[10] The wave did little mischief, the winds being easterly.
_DEEP-SEA DREDGINGS._
Men have ever been strangely charmed by the unknown and the seemingly
inaccessible. The astronomer exhibits the influence of this charm as
he constructs larger and larger telescopes, that he may penetrate more
and more deeply beyond the veil which conceals the greater part of the
universe from the unaided eye. The geologist, seeking to piece together
the fragmentary records of the past which the earth’s surface presents
to him, is equally influenced by the charm of mystery and difficulty.
And the microscopist who tries to force from nature the secret of the
infinitely little, is led on by the same strange desire to discover
just those matters which nature has been most careful to conceal from
us.
The energy with which in recent times men have sought to master the
problem of deep-sea sounding and deep-sea dredging is, perhaps, one
of the most striking instances ever afforded of the charm which the
unknown possesses for mankind. Not long ago, one of the most eminent
geographers of the sea spoke regretfully about the small knowledge
men have obtained of the depths of ocean. ‘Greater difficulties,’ he
remarked, ‘than any presented by the problem of deep-sea research
have been overcome in other branches of physical inquiry. Astronomers
have measured the volumes and weighed the masses of the most distant
planets, and increased thereby the stock of human knowledge. Is it
creditable to the age that the depths of the sea should remain in the
category of unsolved problems? that its “ooze and bottom” should be a
sealed volume, rich with ancient and eloquent legends and suggestive of
many an instructive lesson that might be useful and profitable to man?‘
Since that time, however, deep-sea dredging has gradually become more
and more thoroughly understood and mastered. When the telegraphic
cable which had lain so many months at the bottom of the Atlantic was
hauled on board the ‘Great Eastern’ from enormous depths, men were
surprised and almost startled by the narrative. The appearance of the
ooze-covered cable as it was slowly raised towards the surface, and
the strange thrill which ran through those who saw it and remembered
through what mysterious depths it had twice passed; its breaking away
almost from the very hands of those who sought to draw it on board; and
the successful renewal of the attempt to recover the cable,—all these
things were heard of as one listens to a half-incredible tale. Yet when
that work was accomplished deep-sea dredging had already been some time
a science, and many things had been achieved by its professors which
presented, in reality, greater practical difficulties than the recovery
of the Atlantic Cable.
Recently, however, deep-sea researches have been carried on with
results which are even more sensational, so to speak, than the
grappling feat which so surprised us. Seas so deep that many of the
loftiest summits of the Alps might be completely buried beneath them
have been explored. Dredges weighing with their load of mud nearly half
a ton have been hauled up without a hitch from depths of some 14,000
feet. But not merely has comparatively rough work of this sort been
achieved, but by a variety of ingenious contrivances men of science
have been able to measure the temperature of the sea at depths where
the pressure is so enormous as to be equivalent to a weight of more
than 430 tons on every square foot of surface.
The results of these researches are even more remarkable and
surprising, however, than the means by which they have been obtained.
Sir Charles Lyell has fairly spoken of them as so astonishing ‘that
they have to the geologist almost a revolutionary character.’ Let us
consider a few of them.
No light can be supposed to penetrate to the enormous depth just
spoken of. Therefore, how certainly we might conclude that there can
be no life there. If, instead of dealing with the habitability of
planets, Whewell, in his ‘Plurality of Worlds,’ had been considering
the question whether at depths of two or three miles living creatures
could subsist, how convincingly would he have proved the absurdity of
such a supposition. Intense cold, perfect darkness, and a persistent
pressure of two or three tons to the square inch,—such, he might have
argued, are the conditions under which life exists, if at all, in those
dismal depths. And even if he had been disposed to concede the bare
possibility that life of some sort may be found there, then certainly,
he would have urged, some new sense must replace sight—the creatures in
these depths can assuredly have no eyes, or only rudimentary ones.
But the recent deep-sea dredgings have proved that not only does life
exist in the very deepest parts of the Atlantic, but that the beings
which live and move and have their being beneath three miles of water
have eyes which the ablest naturalists pronounce to be perfectly
developed. Light, then, of some sort must exist in those abysms, though
whether the home of the deep-sea animals be phosphorescent, as Sir
Charles Lyell suggests, or whether light reaches these creatures in
some other way, we have no present means of determining.
If there is one theory which geologists have thought more justly
founded than all others, it is the view that the various strata of the
earth were formed at different times. A chalk district, for example,
lying side by side with a sandstone district, has been referred to a
totally different era. Whether the chalk was formed first, or whether
the sandstone existed before the minute races came into being which
formed the cretaceous stratum, might be a question. But no doubt
existed in the minds of geologists that each formation belonged to a
distinct period. Now, however, Dr. Carpenter and Professor Thomson
may fairly say, ‘We have changed all this.’ It has been found that at
points of the sea-bottom only eight or ten miles apart, there may be
in progress the formation of a cretaceous deposit and of a sandstone
region, each with its own proper fauna. ‘Wherever similar conditions
are found upon the dry land of the present day,’ remarks Dr. Carpenter,
‘it has been supposed that the formation of chalk and the formation of
sandstone must have been separated from each other by long periods,
and the discovery that they may actually co-exist upon adjacent
surfaces has done no less than strike at the very root of the customary
assumptions with regard to geological time.’[11]
Even more interesting, perhaps, to many, are the results which have
been obtained respecting the varying temperatures of deep-sea regions.
The peculiarity just considered is, indeed, a consequence of such
variations; but the fact itself is at least as interesting as the
consequences which flow from it. It throws light on the long-standing
controversy respecting the oceanic circulation. It has been found that
the depths of the equatorial and tropical seas are colder than those
of the North Atlantic. In the tropics the deep-sea temperature is
considerably below the freezing-point of fresh water; in the deepest
part of the Bay of Biscay the temperature is several degrees above the
freezing-point. Thus one learns that the greater part of the water
which lies deep below the surface of the equatorial and tropical seas
comes from the Antarctic regions, though undoubtedly there are certain
relatively narrow currents which carry the waters of the Arctic seas
to the tropics. The great point to notice is that the water under
the equatorial seas must really have travelled from polar regions. A
cold of 30 degrees can be explained in no other way. We see at once,
therefore, the explanation of those westerly equatorial currents which
have been so long a subject of contest. Sir John Herschel failed to
prove that they are due to the trade winds, but Maury failed equally to
prove that they are due to the great warmth and consequent buoyancy of
the equatorial waters. In fact, while Maury showed very convincingly
that the great system of oceanic circulation is carried on _despite_
the winds, Herschel proved in an equally convincing manner that the
overflow conceived by Maury should result in an easterly instead of
a westerly current. Recently the theory was put forward that the
continual process of evaporation going on in the equatorial regions
leads to an indraught of cold water in bottom-currents from the polar
seas. Such currents coming _towards_ the equator, that is, travelling
from latitudes where the earth’s eastwardly motion is less to latitudes
where that motion is greater, would lag behind, that is, would have a
westwardly motion. It seems now placed beyond a doubt that this is the
true explanation of the equatorial ocean-currents.
Such are a few, and but a few, among the many interesting results which
have followed from the recent researches of Dr. Carpenter and Professor
Thomson into the hitherto little-known depths of the great sea.
(From the _Spectator_, December 4, 1869.)
FOOTNOTES:
[11] This opinion Dr. Carpenter has since somewhat modified. It will
be remembered, of course, that the evidence derived from the nature of
superposed strata is in no way affected by what is shown above to hold
with adjacent deposits.
_THE TUNNEL THROUGH MONT CENIS._
Men flash their messages across mighty continents and beneath the bosom
of the wide Atlantic; they weigh the distant planets, and analyse
sun and stars; they span Niagara with a railway bridge, and pierce
the Alps with a railway tunnel: yet the poet of the age in which all
these things are done or doing sings, ‘We men are a puny race.’ And
certainly, the great works which belong to man as a race can no more
be held to evidence the importance of the individual man than the vast
coral reefs and atolls of the Pacific can be held to evidence the
working power of the individual coral polype. But if man, standing
alone, is weak, man working according to the law assigned to his race
from the beginning—that is, in fellowship with his kind—is verily a
being of power.
Perhaps no work ever undertaken by man strikes one as more daring than
the attempt to pierce the Alps with a tunnel. Nature seems to have
upreared these mighty barriers as if with the design of showing man how
weak he is in her presence. Even the armies of Hannibal and Napoleon
seemed all but powerless in the face of these vast natural fastnesses.
Compelled to creep slowly and cautiously along the difficult and
narrow ways which alone were open to them, decimated by the chilling
blasts which swept the face of the rugged mountain-range, and dreading
at every moment the pitiless swoop of the avalanche, the French and
Carthaginian troops exhibited little of the pomp and dignity which we
are apt to associate with the operations of warlike armies. Had the
denizen of some other planet been able to watch their progress, he
might indeed have said ‘these men are a puny race.’ In _this_ only,
that _they succeeded_, did the troops of Hannibal and Napoleon assert
the dignity of the human race. Grand as was the aspect of nature, and
mean as was that of man during the progress of the contest, it was
nature that was conquered, man that overcame.
And now man has entered on a new conflict with nature in the gloomy
fastnesses of the Alps. The barrier which he had scaled of old he has
now undertaken to pierce. And the wwww—bold and daring as it seemed—is
three parts finished. (See date of article.)
The Mont Cenis tunnel was sanctioned by the Sardinian Government in
1857, and arrangements were made for fixing the perforating machinery
in the years 1858 and 1859. But the work was not actually commenced
until November 1860. The tunnel—which will be fully seven and a half
miles in length—was to be completed in twenty-five years. The entrance
to the tunnel on the side of France is near the little village of
Fourneau, and lies 3,946 feet above the level of the sea. The entrance
on the side of Italy is in a deep-valley at Bardonèche, and lies 4,380
feet above the sea level. Thus there is a difference of level of 434
feet. But the tunnel will actually rise 445 feet above the level of the
French end, attaining this height at a distance of about four miles
from that extremity; in the remaining three and three-quarter miles
there will be a fall of only ten feet, so that this part of the line
will be practically level.
The rocks through which the excavations have been made have been for
the most part very difficult to work. Those who imagine that the
great mass of our mountain ranges consists of such granite as is made
use of in our buildings, and is uniform in texture and hardness,
greatly underrate the difficulties with which the engineers of this
gigantic work have had to contend. A large part of the rock consists
of a crystallised calcareous schist, much broken and contorted;
and through this rock run in every direction large masses of pure
quartz. It will be conceived how difficult the work has been of
piercing through so diversified a substance as this. The perforating
machines are calculated to work best when the resistance is uniform;
and it has often happened that the unequal resistance offered to the
perforators has resulted in injury to the chisels. But before the work
of perforating began, enormous difficulties had to be contended with.
It will be understood that, in a tunnel of such vast length, it was
absolutely necessary that the perforating processes carried on from
the two ends should be directed with the most perfect accuracy. It has
often happened in short tunnels that a want of perfect coincidence has
existed between the two halves of the work, and the tunnellers from one
end have sometimes altogether failed to meet those from the other. In
a short tunnel this want of coincidence is not very important, because
the two interior ends of the tunnellings cannot in any case be far
removed from each other. But in the case of the Mont Cenis tunnel any
inaccuracy in the direction of the two tunnellings would have been
fatal to the success of the work, since when the two ought to meet
it might be found that they were laterally separated by two or three
hundred yards. Hence it was necessary before the work began to survey
the intermediate country, so as to ascertain with the most perfect
accuracy the bearings of one end of the tunnel from the other. ‘It was
necessary,’ says the narrative of these initial labours, ‘to prepare
accurate plans and sections for the determination of the levels, to
fix the axis of the tunnel, and to “set it out” on the mountain top;
to erect observatories and guiding signals, solid, substantial, and
true.’ When we remember the nature of the passes over the Cenis, we
can conceive the difficulty of setting out a line of this sort over
the Alpine range. The necessity of continually climbing over rocks,
ravines, and precipices in passing from station to station involved
difficulties which, great as they were, were as nothing when compared
with the difficulties resulting from the bitter weather experienced
on those rugged mountain heights. The tempests which sweep the Alpine
passes—the ever-recurring storms of rain, sleet, and driving snow, are
trying to the ordinary traveller. It will be understood, therefore, how
terribly they must have interfered with the delicate processes involved
in surveying. It often happened that for days together no work of any
sort could be done owing to the impossibility of using levels and
theodolites when exposed to the stormy weather and bitter cold of these
lofty passes. At length, however, the work was completed, and that with
such success that the greatest deviation from exactitude was less than
a single foot for the whole length of seven and a half miles.
Equally remarkable and extensive were the labours connected with the
preparatory works. New and solid roads, bridges, canals, magazines,
workshops, forges, furnaces, and machinery had to be constructed;
residences had to be built for the men, and offices for the engineers;
in fact, at each extremity of the tunnel a complete establishment had
to be formed. Those who have traversed Mont Cenis since the works
began have been perplexed by the strange appearance and character of
the machinery and establishments to be seen at Modane and Fourneau. The
mass of pipes and tubes, tanks, reservoirs, and machinery, which would
be marvellous anywhere, has a still stranger look in a wild and rugged
Alpine pass.
(From the _Daily News_, 1869.)
_TORNADOES._
The inhabitants of the earth are subjected to agencies which—beneficial
doubtless in the long run, perhaps necessary to the very existence of
terrestrial races—appear, at first sight, energetically destructive.
Such are—in order of destructiveness—the hurricane, the earthquake,
the volcano, and the thunderstorm. When we read of earthquakes such as
those which overthrew Lisbon, Callao, and Riobamba, and learn that one
hundred thousand persons fell victims in the great Sicilian earthquake
in 1693, and probably three hundred thousand in the two earthquakes
which assailed Antioch in the years 526 and 612, we are disposed to
assign at once to this devastating phenomenon the foremost place
among the agents of destruction. But this judgment must be reversed
when we consider that earthquakes—though so fearfully and suddenly
destructive both to life and property—yet occur but seldom compared
with wind-storms, while the effects of a real hurricane are scarcely
less destructive than those of the sharpest shocks of earthquakes.
After ordinary storms, long miles of the sea-coast are strewn with
the wrecks of many once gallant ships, and with the bodies of their
hapless crews. In the spring of 1866 there might be seen at a single
view from the heights near Plymouth twenty-two shipwrecked vessels, and
this after a storm which, though severe, was but trifling compared with
the hurricanes which sweep over the torrid zones, and thence—scarcely
diminished in force—as far north sometimes as our own latitudes. It
was in such a hurricane that the ‘Royal Charter’ was wrecked, and
hundreds of stout ships with her. In the great hurricane of 1780,
which commenced at Barbadoes and swept across the whole breadth of the
North Atlantic, fifty sails were driven ashore at the Bermudas, two
line-of-battle ships went down at sea, and upwards of twenty thousand
persons lost their lives on the land. So tremendous was the force of
this hurricane (Captain Maury tells us) that ‘the bark was blown from
the trees, and the fruits of the earth destroyed; the very bottom
and depths of the sea were uprooted—forts and castles were washed
away, and their great guns carried in the air like chaff; houses were
razed; ships wrecked; and the bodies of men and beasts lifted up in
the air and dashed to pieces in the storm’—an account, however, which
(though doubtless faithfully rendered by Maury from the authorities he
consulted) must perhaps be accepted _cum grano_, and especially with
reference to the great guns carried in the air ‘like chaff.’[12] (If
so, it ‘blew great guns,’ indeed.)
In the gale of August, 1782, all the trophies of Lord Rodney’s victory,
except the ‘Ardent,’ were destroyed, two British ships-of-the-line
foundered at sea, numbers of merchantmen under Admiral Graves’ convoy
were wrecked, and at sea alone three thousand lives were lost.
But quite recently a storm far more destructive than these swept over
the Bay of Bengal. Most of my readers doubtless remember the great gale
of October 1864, in which all the ships in harbour at Calcutta were
swept from their anchorage, and driven one upon another in inextricable
confusion. Fearful as was the loss of life and property in Calcutta
harbour, the destruction on land was greater. A vast wave swept for
miles over the surrounding country, embankments were destroyed, and
whole villages, with their inhabitants, were swept away. Fifty thousand
souls, it is believed, perished in this fearful hurricane.
The gale which has just ravaged the Gulf of Mexico adds another to
the long list of disastrous hurricanes. As I write, the effects
produced by this tornado are beginning to be made known. Already its
destructiveness has become but too certainly evidenced.
The laws which appear to regulate the generation and the progress of
cyclonic storms are well worthy of careful study.
The regions chiefly infested by hurricanes are the West Indies, the
southern parts of the Indian Ocean, the Bay of Bengal, and the China
Seas. Each region has its special hurricane season.
In the West Indies, cyclones occur principally in August and September,
when the south-east monsoons are at their height. At the same season
the African south-westerly monsoons are blowing. Accordingly there are
two sets of winds, both blowing heavily and steadily from the Atlantic,
disturbing the atmospheric equilibrium, and thus in all probability
generating the great West Indian hurricanes. The storms thus arising
show their force first at a distance of about six or seven hundred
miles from the equator, and far to the east of the region in which they
attain their greatest fury. They sweep with a north-westerly course to
the Gulf of Mexico, pass thence northwards, and so to the north-east,
sweeping in a wide curve (resembling the letter ∪ placed thus ⊂) around
the West Indian seas, and thence travelling across the Atlantic,
generally expending their fury before they reach the shores of Western
Europe. This course is the storm-track (or storm-⊂ as I shall call it).
Of the behaviour of the winds as they traverse this track, I shall
have to speak when I come to consider the peculiarity from which these
storms derive their names of ‘cyclones’ and tornadoes.
The hurricanes of the Indian Ocean occur at the ‘changing of the
monsoons.’ ‘During the interregnum,‘ writes Maury, ‘the fiends of the
storm hold their terrific sway.’ Becalmed often for a day or two,
seamen hear moaning sounds in the air, forewarning them of the coming
storm. Then, suddenly, the winds break loose from the forces which have
for a while controlled them, and ‘seem to rage with a fury that would
break up the fountains of the deep.’
In the North Indian seas hurricanes rage at the same season as in the
West Indies.
In the China seas occur those fearful gales known among sailors as
‘typhoons’ or ‘white squalls.’ These take place at the changing of the
monsoons. Generated, like the West Indian hurricanes, at a distance of
some ten or twelve degrees from the equator, typhoons sweep—in a curve
similar to that followed by the Atlantic storms—around the East Indian
Archipelago, and the shores of China, to the Japanese Islands.
There occur land-storms, also, of a cyclonic character in the valley
of the Mississippi. ‘I have often observed the paths of such storms,’
says Maury, ‘through the forests of the Mississippi. There the track
of these tornadoes is called a “wind-road,” because they make an
avenue through the wood straight along, and as clear of trees as if
the old denizens of the forest had been cleared with an axe. I have
seen trees three or four feet in diameter torn up by the roots, and
the top, with its limbs, lying next the hole whence the root came.‘
Another writer, who was an eye-witness to the progress of one of these
American land-storms, thus speaks of its destructive effects. ‘I saw,
to my great astonishment, that the noblest trees of the forest were
falling into pieces. A mass of branches, twigs, foliage, and dust moved
through the air, whirled onward like a cloud of feathers, and passing,
disclosed a wide space filled with broken trees, naked stumps, and
heaps of shapeless ruins, which marked the path of the tempest.’
If it appeared, on a careful comparison of observations made in
different places, that these winds swept directly along those tracks
which they appear to follow, a comparatively simple problem would be
presented to the meteorologist. But this is not found to be the case.
At one part of a hurricane’s course the storm appears to be travelling
with fearful fury along the true storm-⊂; at another less furiously
directed across the storm-track; at another, but with yet diminished
force, though still fiercely, in a direction exactly opposite to that
of the storm-track.
All these motions appear to be fairly accounted for by the theory that
the true path of the storm is a spiral—or rather, that while the centre
of disturbance continually travels onwards in a widely extended curve,
the storm-wind sweeps continually around the centre of disturbance, as
a whirlpool around its vortex.
And here a remarkable circumstance attracts our notice, the
consideration of which points to the mode in which cyclones may be
conceived to be generated. It is found, by a careful study of different
observations made upon the same storm, that cyclones in the northern
hemisphere _invariably_ sweep round the onward travelling vortex of
disturbance in _one_ direction, and southern cyclones in the contrary
direction. If we place a watch, face upwards, upon one of the northern
cyclone regions in a Mercator’s chart, then the motion of the hands is
_contrary_ to the direction in which the cyclone whirls; when the watch
is shifted to a southern cyclone region, the motion of the hands is in
the same direction as the cyclone motion. This peculiarity is converted
into the following rule-of-thumb for sailors who encounter a cyclone,
and seek to escape from the region of fiercest storm:—_Facing the wind,
the centre or vortex of the storm lies to the right in the northern,
to the left in the southern hemisphere_. Safety lies in flying from
the centre in every case save one—that is, when the sailor lies in
the direct track of the advancing vortex. In this case, to fly from
the centre would be to keep in the storm-track; the proper course for
the sailor when thus situated is to steer for the calmer side of the
storm-track. This is always the outside of the ⊃, as will appear from
a moment’s consideration of the spiral curve traced out by a cyclone.
Thus, if the seaman _scud before the wind_—in all other cases a
dangerous expedient in a cyclone[13]—he will probably escape unscathed.
There is, however, this danger, that the storm-track may extend to or
even slightly overlap the land, in which case scudding before the gale
would bring the ship upon a lee-shore. And in this way many gallant
ships have, doubtless, suffered wreck.
The danger of the sailor is obviously greater, however, when he is
overtaken by the storm on the inner side of the storm-⊂. Here he has to
encounter the double force of the cyclonic whirl and of the advancing
storm-system, instead of the difference of the two motions, as on the
outer side of the storm-track. His chance of escape will depend on his
distance from the central path of the cyclone. If near to this, it is
equally dangerous for him to attempt to scud to the safer side of the
track, or to beat against the wind by the shorter course, which would
lead him out of the storm-⊂ on its inner side. It has been shown by
Colonel Sir W. Reid that this is the quarter in which vessels have been
most frequently lost.
But even the danger of this most dangerous quarter admits of degrees.
It is greatest where the storm is sweeping round the most curved part
of its track, which happens in about latitude twenty-five or thirty
degrees. In this case a ship may pass twice through the vortex of the
storm. Here hurricanes have worked their most destructive effects. And
hence it is that sailors dread, most of all, that part of the Atlantic
near Florida and the Bahamas, and the region of the Indian Ocean which
lies south of Bourbon and Mauritius.
To show how important it is that captains should understand the theory
of cyclones in both hemispheres, we shall here relate the manner in
which Captain J. V. Hall escaped from a typhoon of the China seas.
About noon, when three days out from Macao, Captain Hall saw ‘a most
wild and uncommon-looking halo round the sun.’ On the afternoon of the
next day, the barometer had commenced to fall rapidly; and though, as
yet, the weather was fine, orders were at once given to prepare for a
heavy gale. Towards evening a bank of cloud was seen in the south-east,
but when night closed the weather was still calm and the water
smooth, though the sky looked wild and a scud was coming on from the
north-east. ‘I was much interested,’ says Captain Hall, ‘in watching
for the commencement of the gale, which I now felt sure was coming.
That bank to the south-east was the meteor (cyclone) approaching us,
the north-east scud, the outer north-west portion of it; and when at
night a strong gale came on about north, or north-north-west, I felt
certain we were on its western and south-western verge. It rapidly
increased in violence; but I was pleased to see the wind veering
to the north-west, as it convinced me that I had put the ship on
the right track—namely, on the starboard tack, standing, of course,
to the south-west. From ten A.M. to three P.M. it blew with great
violence, but the ship being well prepared, rode comparatively easy.
The barometer was now very low, the centre of the storm passing to the
northward of us, to which we might have been very near had we in the
first place put the ship on the larboard tack.
But the most remarkable point of Captain Hall’s account remains to
be mentioned. He had gone out of his course to avoid the storm, but
when the wind fell to a moderate gale he thought it a pity to lie so
far from his proper course, and made sail to the north-west. ‘In less
than two hours the barometer again began to fall and the storm to rage
in heavy gusts.’ He bore again to the south-east, and the weather
rapidly improved. There can be little doubt that but for Captain Hall’s
knowledge of the law of cyclones, his ship and crew would have been
placed in serious jeopardy, since in the heart of a Chinese typhoon a
ship has been known to be thrown on her beam-ends when not showing a
yard of canvas.
If we consider the regions in which cyclones appear, the paths they
follow, and the direction in which they whirl, we shall be able to form
an opinion as to their origin. In the open Pacific Ocean (as its name,
indeed, implies) storms are uncommon; they are infrequent also in the
South Atlantic and South Indian Oceans. Around Cape Horn and the Cape
of Good Hope heavy storms prevail, but they are not cyclonic, nor are
they equal in fury and frequency, Maury tells us, to the true tornado.
Along the equator, and for several degrees on either side of it,
cyclones are also unknown. If we turn to a map in which ocean-currents
are laid down, we shall see that in every ‘cyclone region’ there is
a strongly marked current, and that each current follows closely the
track which I have denominated the storm-⊂. In the North Atlantic we
have the great Gulf Stream, which sweeps from equatorial regions
into the Gulf of Mexico, and thence across the Atlantic to the shores
of Western Europe. In the South Indian Ocean there is the ‘south
equatorial current,’ which sweeps past Mauritius and Bourbon, and
thence returns towards the east. In the Chinese Sea there is the north
equatorial current, which sweeps round the East Indian Archipelago, and
then merges into the Japanese current. There is also the current in the
Bay of Bengal, flowing through the region in which, as we have seen,
cyclones are commonly met with. There are other sea-currents besides
these which yet breed no cyclones. But I may notice two peculiarities
in the currents I have named. They all flow from equatorial to
temperate regions, and, secondly, they are all ‘horse-shoe currents.’
So far as I am aware, there is but one other current which presents
both these peculiarities—namely, the great Australian current between
New Zealand and the eastern shores of Australia. I have not yet met
with any record of cyclones occurring over the Australian current,
but heavy storms are known to prevail in that region, and I believe
that when these storms have been studied as closely as the storms in
better-known regions, they will be found to present the true cyclonic
character.
Now, if we inquire why an ocean current travelling from the equator
should be a ‘storm-breeder,’ we shall find a ready answer. Such a
current, carrying the warmth of intertropical regions to the temperate
zones, produces, in the first place, by the mere difference of
temperature, important atmospheric disturbances. The difference is so
great, that Franklin suggested the use of the thermometer in the North
Atlantic Ocean as a ready means of determining the longitude, since the
position of the Gulf Stream at any given season is almost constant.
But the warmth of the stream itself is not the only cause of
atmospheric disturbance. Over the warm water vapour is continually
rising; and, as it rises, is continually condensed (like the steam
from a locomotive) by the colder air round. ‘An observer on the moon,’
says Captain Maury, ‘would, on a winter’s day, be able to trace out by
the mist in the air the path of the Gulf Stream through the sea.’ But
what must happen when vapour is condensed? We know that to turn water
into vapour is a process requiring—that is, _using up_—a large amount
of heat; and, conversely, the return of vapour to the state of water
_sets free_ an equivalent quantity of heat. The amount of heat thus
set free over the Gulf Stream is thousands of times greater than that
which would be generated by the whole coal supply annually raised in
Great Britain. Here, then, we have an efficient cause for the wildest
hurricanes. For, along the whole of the Gulf Stream, from Bemini to
the Grand Banks, there is a channel of heated—that is, _rarefied air_.
Into this channel, the denser atmosphere on both sides is continually
pouring, with greater or less strength. When a storm begins in the
Atlantic, it always makes for this channel, ‘and, reaching it, turns
and follows it in its course, sometimes entirely across the Atlantic.’
‘The southern points of America and Africa have won for themselves,’
says Maury, ‘the name of “the stormy capes,” but there is not a
storm-find in the wide ocean can out-top that which rages along the
Atlantic coasts of North America. The China seas and the North Pacific
may vie in the fury of their gales with this part of the Atlantic, but
Cape Horn and the Cape of Good Hope cannot equal them, certainly in
frequency, nor do I believe, in fury.’ We read of a West Indian storm
so violent, that ‘it forced the Gulf Stream back to its sources, and
piled up the water to a height of thirty feet in the Gulf of Mexico.
The ship “Ledbury Snow” attempted to ride out the storm. When it abated
she found herself high up on the dry land, and discovered that she had
let go her anchor among the tree-tops on Elliot’s Key.‘
By a like reasoning, we can account for the cyclonic storms prevailing
in the North Pacific Ocean. Nor do the tornadoes which rage in parts
of the United States present any serious difficulty. The region along
which these storms travel is the valley of the great Mississippi. This
river at certain seasons is considerably warmer than the surrounding
lands. From its surface, also, aqueous vapour is continually being
raised. When the surrounding air is colder, this vapour is presently
condensed, generating in the change a vast amount of heat. We have thus
a channel of rarefied air over the Mississippi valley, and this channel
becomes a storm-track, like the corresponding channels over the warm
ocean-currents. The extreme violence of land-storms is probably due to
the narrowness of the track within which they are compelled to travel.
For it has been noticed that the fury of a sea-cyclone increases as the
range of the ‘whirl’ diminishes, and _vice versâ_.
There seems, however, no special reason why cyclones should follow
the storm-⊂ in one direction rather than in the other. We must, to
understand this, recall the fact that under the torrid zones the
conditions necessary for the generation of storms prevail far more
intensely than in temperate regions. Thus the probability is far
greater that cyclones should be generated at the tropical than at the
temperate end of the storm-⊂. Still, it is worthy of notice, that in
the land-locked North Pacific Ocean, true typhoons _have_ been noticed
to follow the storm-track in a direction contrary to that commonly
noticed.
The direction in which a true tornado _whirls_ is _invariably_ that I
have mentioned. The explanation of this peculiarity would occupy more
space than I can here afford. Those readers who may wish to understand
the origin of the law of cyclonic rotation should study Herschel’s
interesting work on Meteorology.
The suddenness with which a true tornado works destruction was
strikingly exemplified in the wreck of the steamship ‘San Francisco.’
She was assailed by an extra-tropical tornado when about 300 miles
from Sandy Hook, on December 24, 1853. In a few moments she was a
complete wreck! The wide range of a tornado’s destructiveness is shown
by this, that Colonel Reid tells us of one along whose track no less
than 110 ships were wrecked, crippled, or dismasted.
(From _Temple Bar_, December 1867.)
FOOTNOTES:
[12] I remember to have read that in this hurricane guns which had long
lain under water were washed up like mere drift upon the beach. Perhaps
this circumstance grew gradually into the incredible story above
recorded.
[13] A ship by scudding before the gale may—if the captain is not
familiar with the laws of cyclones—go _round and round_ without
escaping. The ship ‘Charles Heddle’ did this in the East Indies, going
round no less than _five times_.
_VESUVIUS._
The numerous and violent eruptions from Mount Vesuvius during the two
last centuries seem to afford an answer to those who think there are
traces of a gradually diminishing activity in the earth’s internal
forces. That such a diminution is taking place, we may admit; but
that its rate of progress is perceptible—that we can point to a time
within the historical epoch, nay, even within the limits of geological
evidence, at which the earth’s internal forces were _certainly_ more
active than they are at the present time—may, I think, be denied
absolutely.
When the science of geology was but young, and its professors sought
to compress within a few years (at the outside) a series of events
which (we now know) must have occupied many centuries, there was room,
indeed, for the supposition that modern volcanic eruptions, as compared
with ancient outbursts, are but as the efforts of children compared
with the work of giants. And accordingly, we find a distinguished
French geologist writing, even so late as 1829, that in ancient times
‘tous les phénomènes géologiques se passaient dans des dimensions
_centuples_ de celles qu’ils présentent aujourd’hui.’ But now we have
such certain evidence of the enormous length of the intervals within
which volcanic regions assumed their present appearance—we have such
satisfactory means of determining which of the events occurring within
those intervals were or were not contemporary—that we are safe from
the error of assuming that Nature at a single effort fashioned widely
extended districts just as we now see them. And accordingly, we have
the evidence of the distinguished geologist, Sir Charles Lyell, that
there is no volcanic mass ‘of ancient date, distinctly referable to a
single eruption, which can even _rival_ in volume the matter poured out
from Skaptâr Jokul in 1783.’
In the volcanic region of which Vesuvius or Somma is the principal
vent, we have a remarkable instance of the deceptive nature of that
state of rest into which some of the principal volcanoes frequently
fall for many centuries together. For how many centuries before the
Christian era Vesuvius had been at rest is not known; but this is
certain, that from the landing of the first Greek colony in Southern
Italy, Vesuvius gave no signs of internal activity. It was recognised
by Strabo as a volcanic mountain, but Pliny did not include it in the
list of active volcanoes. In those days, the mountain presented a very
different appearance from that which it now exhibits. In place of the
two peaks now seen, there was a single, somewhat flattish summit,
on which a slight depression marked the place of an ancient crater.
The fertile slopes of the mountain were covered with well-cultivated
fields, and the thriving cities Herculaneum, Pompeii, and Stabiæ stood
near the base of the sleeping mountain. So little did any thought
of danger suggest itself in those times, that the bands of slaves,
murderers, and pirates which flocked to the standards of Spartacus
found a refuge, to the number of many thousands, within the very crater
itself.
But though Vesuvius was at rest, the region of which Vesuvius is the
main vent was far from being so. The island of Pithecusa (the modern
Ischia) was shaken by frequent and terrible convulsions. It is even
related that Prochyta (the modern Procida) was rent from Pithecusa in
the course of a tremendous upheaval, though Pliny derives the name
Prochyta (or ‘poured forth’) from the supposed fact of this island
having been poured forth by an eruption from Ischia. Far more probably,
Prochyta was formed independently by submarine eruptions, as the
volcanic islands near Santorin have been produced in more recent times.
So fierce were the eruptions from Pithecusa, that several Greek
colonies which attempted to settle on this island were compelled to
leave it. About 380 years before the Christian era, colonists under
King Hiero of Syracuse, who had built a fortress on Pithecusa, were
driven away by an eruption. Nor were eruptions the sole cause of
danger. Poisonous vapours, such as are emitted by volcanic craters
after eruption, appear to have exhaled, at times, from extensive tracts
on Pithecusa, and thus to have rendered the island uninhabitable.
Still nearer to Vesuvius lay the celebrated Lake Avernus. The name
Avernus is said to be a corruption of the Greek word _Aornos_,
signifying ‘without birds,’ the poisonous exhalations from the waters
of the lake destroying all birds which attempted to fly over its
surface. Doubt has been thrown on the destructive properties assigned
by the ancients to the vapours ascending from Avernus. The lake is now
a healthy and agreeable neighbourhood, frequented, says Humboldt, by
many kinds of birds, which suffer no injury whatever even when they
skim the very surface of the water. Yet there can be little doubt that
Avernus hides the outlet of an extinct volcano; and long after this
volcano had become inactive, the lake which concealed its site ‘may
have deserved the appellation of “atri janua Ditis,” emitting, perhaps,
gases as destructive of animal life as those suffocating vapours given
out by Lake Quilotoa, in Quito, in 1797, by which whole herds of cattle
were killed on its shores, or as those deleterious emanations which
annihilated all the cattle in the island of Lancerote, one of the
Canaries, in 1730.‘
While Ischia was in full activity, not only was Vesuvius quiescent, but
even Etna seemed to be gradually expiring, so that Seneca ranks this
volcano among the number of nearly extinguished craters. At a later
epoch, Ælian asserted that the mountain itself was sinking, so that
seamen lost sight of the summit at a less distance across the seas than
of old. Yet within the last two hundred years there have been eruptions
from Etna rivalling, if not surpassing, in intensity the convulsions
recorded by ancient historians.
I shall not here attempt to show that Vesuvius and Etna belong to the
same volcanic system, though there is reason not only for supposing
this to be the case, but for the belief that all the subterranean
regions whose effects have been shown from time to time over the
district extending from the Canaries and Azores, across the whole of
the Mediterranean, and into Syria itself, belong to but one great
centre of internal action. But it is quite certain that Ischia and
Vesuvius are outlets from a single source.
While Vesuvius was dormant, resigning for a while its pretensions to
be the principal vent of the great Neapolitan volcanic system, Ischia,
we have seen, was rent by frequent convulsions. But the time was
approaching when Vesuvius was to resume its natural functions, and with
all the more energy that they had been for a while suspended.
In the year 63 (after Christ) there occurred a violent convulsion
of the earth around Vesuvius, during which much injury was done to
neighbouring cities, and many lives were lost. From this period shocks
of earthquake were felt from time to time for sixteen years. These grew
gradually more and more violent, until it began to be evident that the
volcanic fires were about to return to their main vent. The obstruction
which had so long impeded the exit of the confined matter was not,
however, readily removed, and it was only in August in the year 79,
after numerous and violent internal throes, that the superincumbent
mass was at length hurled forth. Rocks and cinders, lava, sand, and
scoriæ, were propelled from the crater, and spread many miles on every
side of Vesuvius.
We have an interesting account of the great eruption which followed
in a letter from the younger Pliny to the younger Tacitus. The latter
had asked for an account of the death of the elder Pliny, who lost his
life in his eagerness to obtain a near view of the dreadful phenomenon.
‘He was at that time,’ says his nephew, ‘with the fleet under his
command at Misenum. On August 24, about one in the afternoon, my mother
desired him to observe a cloud of very extraordinary size and shape.
He had just returned from taking the benefit of the sun, and, after
bathing himself in cold water, and taking a slight repast, had retired
to his study. He arose at once, and went out upon a height whence he
might more distinctly view this strange phenomenon. It was not at this
distance discernible from what mountain the cloud issued, but it was
found afterwards that it came from Vesuvius. I cannot give a more exact
description of its figure than by comparing it to that of a pine-tree,
for it shot up to a great height in the form of a trunk, which extended
itself at the top into a sort of branches; occasioned, I suppose,
either by a sudden gust of air which impelled it, whose force decreased
as it advanced upwards, or else the cloud itself, being pressed back by
its own weight, expanded in this manner. The cloud appeared sometimes
bright, at others dark and spotted, as it was more or less impregnated
with earth and cinders.’
These extraordinary appearances attracted the curiosity of the elder
Pliny. He ordered a small vessel to be prepared, and started to seek a
nearer view of the burning mountain. His nephew declined to accompany
him, being engaged with his studies. As Pliny left the house, he
received a note from a lady whose house, being at the foot of Vesuvius,
was in imminent danger of destruction. He set out, accordingly, with
the design of rendering her assistance, and also of assisting others,
‘for the villas stood extremely thick upon that lovely coast.’ He
ordered the galleys to be put to sea, and steered directly to the point
of danger, so cool in the midst of the turmoil around ‘as to be able
to make and dictate observations upon the motions and figures of that
dreadful scene.’ As he approached Vesuvius, cinders, pumice-stones, and
black fragments of burning rock, fell on and around the ships. ‘They
were in danger, too, of running aground, owing to the sudden retreat
of the sea; vast fragments, also, rolled down from the mountain and
obstructed all the shore.’ The pilot advising retreat, Pliny made
the noble answer, ‘Fortune befriends the brave,’ and bade him press
onwards to Stabiæ. Here he found his friend Pomponianus in great
consternation, already prepared for embarking and waiting only for
a change in the wind. Exhorting Pomponianus to be of good courage,
Pliny quietly ordered baths to be prepared; and ‘having bathed, sat
down to supper with great cheerfulness, or at least (which is equally
heroic) with all the appearance of it.’ Assuring his friend that the
flames which appeared in several places were merely burning villages,
Pliny presently retired to rest, and ‘being pretty fat,’ says his
nephew, ‘and breathing hard, those who attended without actually heard
him snore.’ But it became necessary to awaken him, for the court
which led to his room was now almost filled with stones and ashes.
He got up and joined the rest of the company, who were consulting on
the propriety of leaving the house, now shaken from side to side by
frequent concussions. They decided on seeking the fields for safety:
and fastening pillows on their heads, to protect them from falling
stones, they advanced in the midst of an obscurity greater than that
of the darkest night-though beyond the limits of the great cloud it
was already broad day. When they reached the shore, they found the
waves running too high to suffer them safely to venture to put out to
sea. Pliny ‘having drunk a draught or two of cold water, lay down on
a cloth that was spread out for him; but at this moment the flames and
sulphurous vapours dispersed the rest of the company, and obliged him
to rise. Assisted by two of his servants, he got upon his feet, but
instantly fell down dead; suffocated, I suppose,’ says his nephew,
‘by some gross and noxious vapour, for he always had weak lungs and
suffered from a difficulty of breathing.’ His body was not found until
the third day after his death, when for the first time it was light
enough to search for him. He was found as he had fallen, ‘and looking
more like a man asleep than dead.’
But even at Misenum there was danger, though Vesuvius is distant
no less than fourteen miles. The earth was shaken with repeated
and violent shocks, ‘insomuch,’ says the younger Pliny, ‘that they
threatened our complete destruction.’ When morning came, the light was
faint and glimmering; the buildings around seemed tottering to their
fall, and, standing on the open ground, the chariots which Pliny had
ordered were so agitated backwards and forwards that it was impossible
to keep them steady, even by supporting them with large stones. The
sea was rolled back upon itself, and many marine animals were left dry
upon the shore. On the side of Vesuvius, a black and ominous cloud,
bursting with sulphurous vapours, darted out long trains of fire,
resembling flashes of lightning, but much larger. Presently the great
cloud spread over Misenum and the island of Capreæ. Ashes fell around
the fugitives. On every side ‘nothing was to be heard but the shrieks
of women and children, and the cries of men: some were calling for
their children, others for their parents, others for their husbands,
and only distinguishing each other by their voices: one was lamenting
his own fate, another that of his family; some wished to die, that they
might escape the dreadful fear of death; but the greater part imagined
that the last and eternal night was come, which was to destroy the
gods and the world together.’ At length a light appeared, which was
not, however, the day, but the forerunner of an outburst of flames.
These presently disappeared, and again a thick darkness spread over
the scene. Ashes fell heavily upon the fugitives, so that they were in
danger of being crushed and buried in the thick layer rapidly covering
the whole country. Many hours passed before the dreadful darkness began
slowly to be dissipated. When at length day returned, and the sun was
seen faintly shining through the overhanging canopy of ashes, ‘every
object seemed changed, being covered over with white ashes as with a
deep snow.’
It is most remarkable that Pliny makes no mention in his letter of
the destruction of the two populous and important cities, Pompeii and
Herculaneum. We have seen that at Stabiæ a shower of ashes fell so
heavily that several days before the end of the eruption the court
leading to the elder Pliny’s room was beginning to be filled up; and
when the eruption ceased, Stabiæ was completely overwhelmed. Far more
sudden, however, was the destruction of Pompeii and Herculaneum.
It would seem that the two cities were first shaken violently by the
throes of the disturbed mountain. The signs of such a catastrophe
have been very commonly assigned to the earthquake which happened in
63, but it seems far more likely that most of them belong to the days
immediately preceding the great outburst in 79. ‘In Pompeii,’ says Sir
Charles Lyell, ‘both public and private buildings bear testimony to
the catastrophe. The walls are rent, and in many places traversed by
fissures still open.’ It is probable that the inhabitants were driven
by these anticipatory throes to fly from the doomed towns. For though
Dion Cassius relates that ‘two entire cities, Herculaneum and Pompeii,
were buried under showers of ashes, while all the people were sitting
in the theatre,’ yet ‘the examination of the two cities enables us to
prove,’ says Sir Charles, ‘that none of the people were destroyed in
the theatre, and, indeed, that there were very few of the inhabitants
who did not escape from both cities. Yet,’ he adds, ‘some lives were
lost, and there was ample foundation for the tale in all its most
essential particulars.’
We may note here, in passing, that the account of the eruption given by
Dion Cassius, who wrote a century and a half after the catastrophe, is
sufficient to prove how terrible an impression had been made upon the
inhabitants of Campania, from whose descendants he in all probability
obtained the materials of his narrative. He writes that, ‘during the
eruption, a multitude of men of superhuman stature, resembling giants,
appeared, sometimes on the mountain, and sometimes in the environs;
that stones and smoke were thrown out, the sun was hidden, and then
the giants seemed to rise again, while the sounds of trumpets were
heard’—with much other matter of a similar sort.
In the great eruption of 79, Vesuvius poured forth lapilli, sand,
cinders, and fragments of old lava, but no new lava flowed from the
crater. Nor does it appear that any lava-stream was ejected during the
six eruptions which took place during the following ten centuries.
In the year 1036, for the first time, Vesuvius was observed to pour
forth a stream of molten lava. Thirteen years later, another eruption
took place; then ninety years passed without disturbance, and after
that a long pause of 168 years. During this interval, however, the
volcanic system, of which Vesuvius is the main but not the only vent,
had been disturbed twice. For it is related that in 1198 the Solfatara
Lake crater was in eruption: and in 1302, Ischia, dormant for at
least 1,400 years, showed signs of new activity. For more than a year
earthquakes had convulsed this island from time to time, and at length
the disturbed region was relieved by the outburst of a lava-stream from
a new vent on the south-east of Ischia. The lava-stream flowed right
down to the sea, a distance of two miles. For two months, this dreadful
outburst continued to rage; many houses were destroyed; and although
the inhabitants of Ischia were not completely expelled, as happened of
old with the Greek colonists, yet a partial emigration took place.
The next eruption of Vesuvius occurred in 1306; and then three
centuries and a quarter passed during which only one eruption, and
that an unimportant one (in 1500), took place. ‘It was remarked,’ says
Sir Charles Lyell, ‘that throughout this long interval of rest, Etna
was in a state of unusual activity, so as to lend countenance to the
idea that the great Sicilian volcano may sometimes serve as a channel
of discharge to elastic fluids and lava that would otherwise rise to
the vents in Campania.’
Nor was the abnormal activity of Etna the only sign that the quiescence
of Vesuvius was not to be looked upon as any evidence of declining
energy in the volcanic system. In 1538 a new mountain was suddenly
thrown up in the Phlegræan Fields—a district including within its
bounds Pozzuoli, Lake Avernus, and the Solfatara. The new mountain was
thrown up near the shores of the Bay of Baiæ. It is 440 feet above
the level of the bay, and its base is about a mile and a half in
circumference. The depth of the crater is 421 feet, so that its bottom
is only six yards above the level of the bay. The spot on which the
mountain was thrown up was formerly occupied by the Lucrine Lake; but
the outburst filled up the greater part of the lake, leaving only a
small and shallow pool.
The accounts which have reached us of the formation of this new
mountain are not without interest. Falconi, who wrote in 1538, mentions
that several earthquakes took place during the two years preceding
the outburst, and above twenty shocks on the day and night before
the eruption. ‘The eruption began on September 29, 1538. It was on
a Sunday, about one o’clock in the night, when flames of fire were
seen between the hot-baths and Tripergola. In a short time the fire
increased to such a degree that it burst open the earth in this place,
and threw up a quantity of ashes and pumice-stones, mixed with water,
which covered the whole country. The next morning the poor inhabitants
of Pozzuoli quitted their habitations in terror, covered with the muddy
and black shower, which continued the whole day in that country—flying
from death, but with death painted in their countenances. Some with
their children in their arms, some with sacks full of their goods;
others leading an ass, loaded with their frightened family, towards
Naples.... The sea had retired on the side of Baiæ, abandoning a
considerable tract; and the shore appeared almost entirely dry, from
the quantity of ashes and broken pumice-stones thrown up by the
eruption.‘
Pietro Giacomo di Toledo gives us some account of the phenomena which
preceded the eruption: ‘That plain which lies between Lake Avernus,
the Monte Barbaro, and the sea, was raised a little, and many cracks
were made in it, from some of which water issued; at the same time the
sea immediately adjoining the plain dried up about two hundred paces,
so that the fish were left on the sand, a prey to the inhabitants of
Pozzuoli. At last, on September 29, about two o’clock in the night, the
earth opened near the lake, and discovered a horrid mouth, from which
were furiously vomited smoke, fire, stones, and mud composed of ashes,
making at the time of the opening a noise like the loudest thunder.
The stones which followed were by the flames converted to pumice,
and some of these were _larger than an ox_. The stones went about as
high as a cross-bow will carry, and then fell down sometimes on the
edge, and sometimes into the mouth itself. The mud was of the colour
of ashes, and at first very liquid, then by degrees less so; and in
such quantities that in less than twelve hours, with the help of the
above-mentioned stones, a mountain was raised of a thousand paces in
height. Not only Pozzuoli and the neighbouring country were full of
this mud, but the city of Naples also; so that many of its palaces were
defaced by it. This eruption lasted two nights and two days without
intermission, though not always with the same force; the third day the
eruption ceased, and I went up with many people to the top of the new
hill, and saw down into its mouth, which was a round cavity about a
quarter of a mile in circumference, in the middle of which the stones
which had fallen were boiling up just as a cauldron of water boils on
the fire. The fourth day it began to throw up again, and the seventh
day much more, but still with less violence than the first night. At
this time many persons who were on the hill were knocked down by the
stones and killed, or smothered with the smoke.’
And now, for nearly a century, the whole district continued in
repose. Nearly five centuries had passed since there had been any
violent eruption of Vesuvius itself; and the crater seemed gradually
assuming the condition of an extinct volcano. The interior of the
crater is described by Bracini, who visited Vesuvius shortly before
the eruption of 1631, in terms that would have fairly represented its
condition before the eruption of 79:—‘The crater was five miles in
circumference, and about a thousand paces deep; its sides were covered
with brushwood, and at the bottom there was a plain on which cattle
grazed. In the woody parts, wild boars frequently harboured. In one
part of the plain, covered with ashes, were three small pools, one
filled with hot and bitter water, another salter than the sea, and a
third hot, but tasteless.’ But in December, 1631, the mountain blew
away the covering of rock and cinders which supported these woods and
pastures. Seven streams of lava poured from the crater, causing a
fearful destruction of life and property. Resina, built over the site
of Herculaneum, was entirely consumed by a raging lava-stream. Heavy
showers of rain, generated by the steam evolved during the eruption,
caused in their turn an amount of destruction scarcely less important
than that resulting from the lava-streams. For, falling upon the cone,
and sweeping thence large masses of ashes and volcanic dust, these
showers produced destructive streams of mud, consistent enough to merit
the name of ‘aqueous lava’ commonly assigned to it.
An interval of thirty-five years passed before the next eruption.
But since 1666 there has been a continual series of eruptions, so
that the mountain has scarcely ever been at rest for more than ten
years together. Occasionally there have been two eruptions within a
few months; and it is well worthy of remark that, during the three
centuries which have elapsed since the formation of Monte Nuovo, there
has been no volcanic disturbance in any part of the Neapolitan volcanic
district save in Vesuvius alone. Of old, as Brieslak well remarks,
there had been irregular disturbances in some part of the Bay of Naples
once in every two hundred years:—the eruption of Solfatara in the
twelfth century, that of Ischia in the fourteenth, and that of Monte
Nuovo in the sixteenth; but ‘the eighteenth has formed an exception to
the rule.’ It seems clear that the constant series of eruptions from
Vesuvius during the past two hundred years has sufficed to relieve the
volcanic district of which Vesuvius is the principal vent.
Of the eruptions which have disturbed Vesuvius during the last two
centuries, those of 1779, 1793, and 1822, are in some respects the most
remarkable.
Sir William Hamilton has given a very interesting account of the
eruption of 1779. Passing over those points in which this eruption
resembled others, we may note its more remarkable features. Sir William
Hamilton says, that in this eruption molten lava was thrown up in
magnificent jets to the height of at least 10,000 feet. Masses of
stones and scoriæ were to be seen propelled along by these lava jets.
Vesuvius seemed to be surmounted by an enormous column of fire. Some
of the jets were directed by the wind towards Ottajano; others fell on
the cone of Vesuvius, on the outer circular mountain Somma, and on
the valley between. Falling, still red-hot and liquid, they covered a
district more than two miles and a half wide with a mass of fire. The
whole space above this district, to the height of 10,000 feet, was
filled also with the falling and rising lava streams; so that there
was continually present a body of fire covering the extensive space I
have mentioned, and extending nearly two miles high. The heat of this
enormous fire-column was distinctly perceptible at a distance of at
least six miles on every side.
The eruption of 1793 presented a different aspect. Dr. Clarke tells us
that millions of red-hot stones were propelled into the air to at least
half the height of the cone itself; then turning, they fell all around
in noble curves. They covered nearly half the cone of Vesuvius with
fire. Huge masses of white smoke were vomited forth by the disturbed
mountain, and formed themselves, at a height of many thousands of feet
above the crater, into a huge, ever-moving canopy, through which, from
time to time, were hurled pitch-black jets of volcanic dust, and dense
vapours, mixed with cascades of red-hot rocks and scoriæ. The rain
which fell from the cloud-canopy was scalding hot.
Dr. Clarke was able to compare the different appearances presented by
the lava where it burst from the very mouth of the crater, and lower
down when it had approached the plain. As it rushed forth from its
imprisonment, it streamed, a liquid, white, and brilliantly pure river,
which burned for itself a smooth channel through a great arched chasm
in the side of the mountain. It flowed with the clearness of ‘honey in
regular channels, cut finer than art can imitate, and glowing with all
the splendour of the sun. Sir William Hamilton had conceived,’ adds Dr.
Clarke, ‘that stones thrown upon a current of lava would produce no
impression. I was soon convinced of the contrary. Light bodies, indeed,
of five, ten, and fifteen pounds’ weight, made little or no impression,
even at the source; but bodies of sixty, seventy, and eighty pounds
were seen to form a kind of bed on the surface of the lava, and float
away with it. A stone of three hundredweight, that had been thrown out
by the crater, lay near the source of the current of lava. I raised
it up on one end, and then let it fall in upon the liquid lava, when
it gradually sank beneath the surface and disappeared. If I wished to
describe the manner in which it acted upon the lava, I should say that
it was like a loaf of bread thrown into a bowl of very thick honey,
which gradually involves itself in the heavy liquid and then slowly
sinks to the bottom.
But as the lava flowed down the mountain slopes it lost its brilliant
whiteness; a crust began to form upon the surface of the still molten
lava, and this crust broke into innumerable fragments of porous matter
called scoriæ. Underneath this crust—across which Dr. Clarke and his
companions were able to pass without other injury than the singeing of
their boots—the liquid lava still continued to force its way onward
and downward past all obstacles. On its arrival at the bottom of the
mountain, says Dr. Clarke, ‘the whole current,’ encumbered with huge
masses of scoriæ, ‘resembled nothing so much as a heap of unconnected
cinders from an iron foundry,’ ‘rolling slowly along,‘ he says in
another place, ‘and falling with a rattling noise over one another.’
After the eruption described by Dr. Clarke, the great crater gradually
filled up. Lava boiled up from below, and small craters, which formed
themselves over the bottom and sides of the great one, poured forth
lava loaded with scoriæ. Thus, up to October 1822, there was to be
seen, in place of a regular crateriform opening, a rough and uneven
surface, scored by huge fissures, whence vapour was continually being
poured, so as to form clouds above the hideous heap of ruins. But the
great eruption of 1822 not only flung forth all the mass which had
accumulated within the crater, but wholly changed the appearance of the
cone. An immense abyss was formed, three-quarters of a mile across, and
extending 2,000 feet downwards into the very heart of Vesuvius. Had the
lips of the crater remained unchanged, indeed, the depth of this great
gulf would have been far greater. But so terrific was the force of the
explosion that the whole of the upper part of the cone was carried
clean away, and the mountain reduced in height by nearly a full fifth
of its original dimensions. From the time of its formation the chasm
gradually filled up; so that, when Mr. Scrope saw it soon after the
eruption, its depth was reduced by more than 1,000 feet.
Of late, Vesuvius has been as busy as ever. In 1833 and 1834 there were
eruptions; and in 1856 another great outburst took place. Then, for
three weeks together, lava streamed down the mountain slopes. A river
of molten lava swept away the village of Cercolo, and ran nearly to
the sea at Ponte Maddaloni. There were then formed ten small craters
within the great one. But these have now united (see date of article),
and pressure from beneath has formed a vast cone where they had been.
The cone has risen above the rim of the crater, from which torrents of
lava are poured forth. At first the lava formed a lake of fire, but
the seething mass found an outlet, and poured in a wide stream towards
Ottajano. Masses of red-hot stone and rock are hurled forth, and a vast
canopy of white vapour hangs over Vesuvius, forming at night, when
illuminated by the raging mass below, a glory of resplendent flame
around the summit of the mountain.
It may seem strange that the neighbourhood of so dangerous a mountain
should be inhabited by races free to choose more peaceful districts.
Yet, though Herculaneum, Pompeii, and Stabiæ lie buried beneath the
lava and ashes thrown forth by Vesuvius, Portici and Resina, Torre
del Greco and Torre dell’ Annunziata have taken their place; and a
large population, cheerful and prosperous, flourishes around the
disturbed mountain, and over the district of which it is the somewhat
untrustworthy safety-valve.
It has, indeed, been well pointed out by Sir Charles Lyell that ‘the
general tendency of subterranean movements, when their effects are
considered for a sufficient lapse of ages, is eminently beneficial,
and that they constitute an essential part of that mechanism by which
the integrity of the habitable surface is preserved. Why the working
of this same machinery should be attended with so much evil, is a
mystery far beyond the reach of our philosophy, and must probably
remain so until we are permitted to investigate, not our planet alone
and its inhabitants, but other parts of the moral and material universe
with which they may be connected. Could our survey embrace other
worlds, and the events, not of a few centuries only, but of periods
as indefinite as those with which geology renders us familiar, some
apparent contradictions might be reconciled, and some difficulties
would doubtless be cleared up. But even then, as our capacities are
finite, while the scheme of the universe must be infinite, both in time
and space, it is presumptuous to suppose that all sources of doubt and
perplexity would ever be removed. On the contrary, they might, perhaps,
go on augmenting in number although our confidence in the wisdom of the
plan of nature might increase at the same time; for it has been justly
said’ (by Sir Humphry Davy) ‘that the greater the circle of light, the
greater the boundary of darkness by which it is surrounded.’
(From the _Cornhill Magazine_, March 1868.)
_THE EARTHQUAKE IN PERU._
The intelligence published last Saturday (see date of article) is
sufficient to prove that the great earthquake which has devastated
Peru fully equalled, if it did not surpass, the most terrible
catastrophes which have ever befallen that country. It presents, too,
all the features which have hitherto characterised earthquakes in this
neighbourhood. These are well worthy of careful study, and appear to
have an important bearing on the modern theory of earthquakes.
It has been commonly held that the seat of disturbance in the
earthquakes which have shaken the country west of the Andes has lain
always at some point or other beneath that range of mountains. The
fact that several large volcanoes are found in the Cordilleras has
seemed confirmatory of this view. The account we have also of the great
earthquake at Riobamba in 1797, seems only explicable by supposing
that the seat of disturbance lay almost immediately beneath that city.
The inhabitants were flung vertically upwards into the air, and to
such a height that Humboldt found the skeletons of many of them on the
summit of the hill La Culca, on the farther side of the small river on
which Riobamba is built. The ruins of many houses were also flung to
the same spot. Here, therefore, was evidence of that vertical (or, as
Humboldt expresses it, explosive) force which is only to be looked for
immediately above the centre of concussion.
Yet the consideration of the evidence afforded by the news just
published seems at first sight somewhat opposed to this view, and
to point rather to a seat of disturbance lying considerably to the
west of the Peruvian shores. ‘At Chala,’ says our informant, ‘the sea
receded, and a wave rose fifty feet, and returned, spreading into the
town, a distance of about a thousand feet. Three successive times
everything within range was swept away, followed by twelve shocks of
earthquake, lasting from three seconds to two minutes.’ The arrival
of great sea-waves before the land-shocks were felt, seems decisively
to indicate that the seat of disturbance lay beneath the ocean, and
not beneath the land. I am disposed to believe, however, that in
the confusion of mind naturally resulting from the occurrence of so
terrible a catastrophe, the sequence of events may not have been very
closely attended to, for in other places the arrival of the great
sea-wave is distinctly described as following the occurrence of the
earth-shock. At Arica, for example, a considerable interval would
seem to have elapsed before the terrible sea-wave, which has always
characterised Peruvian earthquakes, poured in upon the town. The agent
of the Pacific Steam Navigation Company, whose house had been destroyed
by the earth-shock, saw the great sea-wave while he was flying towards
the hills. He writes:—’While passing towards the hills, with the earth
shaking, a great cry went up to heaven. The sea had retired. On
clearing the town, I looked back and saw that the vessels were being
carried irresistibly seawards. In a few minutes the sea stopped, and
then arose a mighty wave fifty feet high, and came in with a fearful
rush, carrying everything before it in terrible majesty. The whole of
the shipping came back, speeding towards inevitable doom. In a few
minutes all was completed—every vessel was either on shore or bottom
upwards.’ This, then, was undoubtedly the great sea-wave, as compared
with the minor waves of disturbance which characterise all earthquakes
near the shores of the ocean.
One remarkable feature in this terrible earthquake is the enormous
range of country affected by it. From Quito southwards as far as
Iquique—or, in other words, for a distance considerably exceeding a
full third part of the whole length of the South American Andes—the
shock was felt with the most terrible distinctness. We have yet to
learn how much farther to the north and south, and how far inland on
the eastern slopes of the Andes, the shock was experienced. But there
can be little doubt that the disturbed country was equal to at least a
fourth of Europe.
The portion of the Andes thus disturbed seems to be distinct from the
part to which the great Chilian earthquakes belong. The difference in
character between the Peruvian and Chilian earthquakes is a singular
and interesting phenomenon. The difference corresponds to a feature
long since pointed out by Sir Charles Lyell,—the alternation, on a
grand scale, of districts of active with those of extinct volcanoes.
It is said that in Chili a year scarcely ever passes without shocks of
earthquake being felt; in certain regions, not even a month. A similar
persistence of earthquake-disturbance characterises Peru. Yet, although
both districts are shaken in this manner, there seems to be distinct
evidence of alternating disturbance as respects the occurrence of
great earthquakes. Thus in 1797 took place the terrible earthquake of
Riobamba. Then, thirty years later, a series of great earthquakes shook
Chili, permanently elevating the whole line of coast to the height of
several feet. Now, again, after another interval of about thirty years,
the Andes are disturbed by a great earthquake, and this time it is the
Peruvian Andes which experience the shock. Between Chili and Peru there
is a space upwards of five hundred miles long, in which no volcanic
action has been observed. Singularly enough, this very portion of the
Andes, to which one would imagine the Peruvians and Chilians would fly
as to a region of safety, is the part most thinly inhabited, insomuch
that, as Von Buch observes, it is in some places entirely deserted.
Near Quito the trembling of the earth is almost incessant, according
to M. Boussingault. He considers that the frequency of the movement
is due rather to the continual falling in of masses of rock which
have been fractured in recent earthquakes, than to the persistence of
subterranean action. He adds that the height of several mountains in
the Andes has diminished in modern times. He refers, doubtless, to the
Peruvian and Columbian Andes, and not to the Chilian. In the latter
portion of the range there must be a continual increase of height,
since each earthquake in Chili has produced a perceptible recession of
the sea. Darwin, indeed, relates that near Valparaiso he saw beds of
seashells belonging to recent species at a height of about a quarter of
a mile above the present sea-level; and he concluded that the land had
been raised to this height by a series of such small elevations as were
observed to have taken place during the earthquakes of 1822, 1835, and
1837. That a contrary process should be going on in Peru, confirms the
idea that a sort of undulatory or balancing motion is taking place—one
long stretch of the Cordilleras rising while another is sinking. A
tradition prevails among the Indians of Lican that the mountain called
L’Altar, or Cassac Urcu—which means ‘the chief’—was once the highest of
the sub-equatorial Andes, being higher even than Chimborazo; but, adds
the tradition, in the reign of Quainia Abomatha, before the discovery
of America, a prodigious eruption took place, which lasted no less
than eight years, and brought down the summit of the mountain. M.
Boussingault states that the fragments of trachyte which once formed
the summit of this celebrated mountain are now spread over the plain.
At present Cotopaxi is the loftiest volcano of the Cordilleras, its
height being no less than 18,858 feet. No mountain has ever been the
seat of such terrible and destructive eruptions as those which have
burst forth from Cotopaxi. The intensity of the heat which prevails
during eruption will be readily gathered from the circumstance that in
January 1803 the enormous bed of snow which usually covers the cone of
the volcano was dissolved in a single night.
It would seem that the Mexican volcanoes also belong to the same region
of disturbance. Near the Isthmus of Panama the great Cordillera of the
Andes is reduced to the height of about 800 feet, and beyond begins
the continuation of the volcanic chain in Central America and Mexico.
Nor are the volcanoes of the West Indian or Caribbee Islands wholly
disconnected with the region of disturbance in Southern America. And
it is rather singular that even the earthquakes which have occurred in
the valley of the Mississippi seem to be connected with the West Indian
and South American volcanic region. The violent earthquakes which
took place at New Madrid in 1812 occurred at exactly the same time as
the earthquake of Paranas, ‘so that it is possible,’ says Sir Charles
Lyell, ‘that these two points are part of one volcanic region.’
(From the _Daily News_, September 18, 1868.)
_THE GREATEST SEA-WAVE EVER KNOWN._
On August 13, 1868, one of the most terrible calamities which has ever
visited a people befell the unfortunate inhabitants of Peru. In that
land earthquakes are nearly as common as rain-storms are with us; and
shocks by which whole cities are changed into a heap of ruins are by
no means infrequent. Yet even in Peru, ‘the land of earthquakes,’ as
Humboldt has termed it, no such catastrophe as that of August 1868 had
occurred within the memory of man. It was not one city which was laid
in ruins, but a whole empire. Those who perished were counted by tens
of thousands, while the property destroyed by the earthquake was valued
at millions of pounds sterling.
Although so many months have passed since this terrible calamity took
place, scientific men have been busily engaged until quite recently
in endeavouring to ascertain the real significance of the various
events which were observed during and after the occurrence of the
earthquake. The geographers of Germany have taken a special interest
in interpreting the evidence afforded by this great manifestation of
nature’s powers. Two papers have been written recently on the great
earthquake of August 13, 1868, one by Professor Von Hochstetter, the
other by Herr Von Tschudi, which present an interesting account of the
various effects, by land and by sea, which resulted from the tremendous
upheaving force to which the western flanks of the Peruvian Andes were
subjected on that day. The effects on land, although surprising and
terrible, yet only differ in degree from those which have been observed
in other earthquakes. But the progress of the great sea-wave which
was generated by the upheaval of the Peruvian shores and propagated
over the whole of the Pacific Ocean differs altogether from any
earthquake-phenomena before observed. Other earthquakes have indeed
been followed by oceanic disturbances; but these have been accompanied
by terrestrial motions, so as to suggest the idea that they had been
caused by the motion of the sea-bottom, or of the neighbouring land.
In no instance has it ever before been known that a well-marked wave
of enormous proportions should have been propagated over the largest
ocean-tract on our globe, by an earth-shock whose direct action was
limited to a relatively small region, and that region not situated in
the centre, but on one side of the wide area traversed by the wave.
I propose to give a brief sketch of the history of this enormous
sea-wave. In the first place, however, it may be well to remind the
reader of a few of the more prominent features of the great shock to
which this wave owed its origin.
It was at Arequipa, at the foot of the lofty volcanic mountain
Misti, that the most terrible effects of the great earthquake
were experienced. Within historic times Misti has poured forth no
lava-streams; but that the volcano is not extinct is clearly shown by
the fact that in 1542 an enormous mass of dust and ashes was vomited
forth from its crater. On August 13, 1868, Misti showed no signs of
being disturbed. So far as their volcanic neighbour was concerned,
the 44,000 inhabitants of Arequipa had no reason to anticipate the
catastrophe which presently befell them. At five minutes past five an
earthquake shock was experienced, which, though severe, seems to have
worked little mischief. Half a minute later, however, a terrible noise
was heard beneath the earth; a second shock more violent than the first
was felt; and then began the swaying motion, gradually increasing in
intensity. In the course of the first minute this motion had become so
violent that the inhabitants ran in terror out of their houses into
the streets and squares. In the next two minutes the swaying movement
had so increased that the more lightly-built houses were cast to the
ground, and the flying people could scarcely keep their feet. ‘And
now,’ says Von Tschudi, ‘there followed during two or three minutes a
terrible scene. The swaying motion which had hitherto prevailed changed
into fierce vertical upheaval. The subterranean roaring increased
in the most terrifying manner: then were heard the heart-piercing
shrieks of the wretched people, the bursting of walls, the crashing
fall of houses and churches, while over all rolled thick clouds of a
yellowish-black dust, which, had they been poured forth many minutes
longer, would have suffocated thousands.’ Although the shock had lasted
but a few minutes, the whole town was destroyed. Not one building
remained uninjured, and there were few which did not lie in shapeless
heaps of ruins.
At Tacna and Arica, the earth-shock was less severe, but strange and
terrible phenomena followed it. At the former place a circumstance
occurred, the cause and nature of which yet remain a mystery. About
three hours after the earthquake—in other words, at about eight o’clock
in the evening—an intensely brilliant light made its appearance above
the neighbouring mountains. It lasted for fully half an hour, and has
been ascribed to the eruption of some as yet unknown volcano.
At Arica the sea-wave produced even more destructive effects than had
been caused by the earthquake. About twenty minutes after the first
earth-shock, the sea was seen to retire, as if about to leave the
shores wholly dry; but presently its waters returned with tremendous
force. A mighty wave, whose length seemed immeasurable, was seen
advancing like a dark wall upon the unfortunate town, a large part of
which was overwhelmed by it. Two ships, the Peruvian corvette ‘America’
and the United States ‘double-ender’ ‘Watertree,‘ were carried
nearly half a mile to the north of Arica, beyond the railroad which
runs to Tacna, and there left stranded high and dry. This enormous wave
was considered by the English vice-consul at Arica to have been fully
fifty feet in height.
At Chala, three such waves swept in after the first shocks of
earthquake. They overflowed nearly the whole of the town, the sea
passing more than half a mile beyond its usual limits.
At Islay and Iquique similar phenomena were manifested. At the former
town the sea flowed in no less than five times, and each time with
greater force. Afterwards the motion gradually diminished, but even an
hour and a half after the commencement of this strange disturbance, the
waves still ran forty feet above the ordinary level. At Iquique, the
people beheld the inrushing wave whilst it was still a great way off. A
dark blue mass of water, some fifty feet in height, was seen sweeping
in upon the town with inconceivable rapidity. An island lying before
the harbour was completely submerged by the great wave, which still
came rushing on, black with the mud and slime it had swept from the
sea-bottom. Those who witnessed its progress from the upper balconies
of their houses, and presently saw its black mass rushing close beneath
their feet, looked on their safety as a miracle. Many buildings were
indeed washed away, and in the low-lying parts of the town there was
a terrible loss of life. After passing far inland the wave slowly
returned seawards, and strangely enough, the sea, which elsewhere
heaved and tossed for hours after the first great wave had swept over
it, here came soon to rest.
At Callao a yet more singular instance was afforded of the effect
which circumstances may have upon the motion of the sea after a
great earthquake has disturbed it. In former earthquakes Callao has
suffered terribly from the effects of the great sea-wave. In fact,
on two occasions the whole town has been destroyed, and nearly all
its inhabitants have been drowned, through the inrush of precisely
such waves as flowed into the ports of Arica and Chala. But upon
this occasion the centre of subterranean disturbance must have been
so situated that either the wave was diverted from Callao, or more
probably two waves reached Callao from different sources and at
different times, so that the two undulations partly counteracted each
other. Certain it is that, although the water retreated strangely from
the coast near Callao, insomuch that a wide tract of the sea-bottom was
uncovered, there was no inrushing wave comparable with those described
above. The sea afterwards rose and fell in an irregular manner, a
circumstance confirming the supposition that the disturbance was caused
by two distinct oscillations. Six hours after the occurrence of the
earth-shock, the double oscillations seem for a while to have worked
themselves into unison, for at this time three considerable waves
rolled in upon the town. But clearly these waves must not be compared
with those which in other instances had made their appearance within
half an hour of the earth-throes. There is little reason to doubt that
if the separate oscillations had reinforced each other earlier, Callao
would have been completely destroyed. As it was, a considerable amount
of mischief was effected; but the motion of the sea presently became
irregular again, and so continued until the morning of August 14th,
when it began to ebb with some regularity. But during the 14th there
were occasional renewals of the irregular motion, and several days
elapsed before the regular ebb and flow of the sea were resumed.
Such were among the phenomena presented in the region where the
earthquake itself was felt. It will be seen at once that within this
region, or rather along that portion of the sea-coast which falls
within the central region of disturbance, the true character of the
sea-wave generated by the earthquake could not be recognised. If a rock
fall from a lofty cliff into a comparatively shallow sea, the water
around the place where the rock has fallen is disturbed in an irregular
manner. The sea seems at one place to leap up and down; elsewhere one
wave seems to beat against another, and the sharpest eye can detect
no law in the motion of the seething waters. But presently, outside
the scene of disturbance, a circular wave is seen to form, and if the
motion of this wave be watched, it is seen to present the most striking
contrast to the turmoil and confusion at its centre. It sweeps onwards
and outwards in a regular undulation. Gradually it loses its circular
figure (unless the sea-bottom happens to be unusually level), showing
that although its motion is everywhere regular, it is not everywhere
equally swift. A wave of this sort, though incomparably vaster, swept
swiftly away on every side from the scene of the great earthquake near
the Peruvian Andes. It has been calculated that the width of this wave
varied from one million to five million feet, or roughly, from 200
to 1,000 miles, while, when in mid-Pacific, the length of the wave,
measured along its summit in a widely-curved path from one side to
another of the great ocean, cannot have been less than 8,000 miles.
We cannot tell how deep-seated was the centre of subterranean action;
but there can be no doubt it was very deep indeed, because otherwise
the shock felt in towns separated from each other by hundreds of miles
could not have been so nearly contemporaneous. Therefore the portion of
the earth’s crust upheaved must have been enormous, for the length of
the region where the direct effects of the earthquake were perceived is
estimated by Professor Von Hochstetter at no less than 240 miles. The
breadth of the region is unknown, because on one side the slope of the
Andes and on the other the ocean concealed the motion of the earth’s
crust.
The great ocean wave swept, as I have said, in all directions around
the scene of the earth-throe. Over a large part of its course its
passage was unnoted, because in the open sea the effects even of so
vast an undulation could not be perceived. A ship would slowly rise as
the crest of the great wave passed under her, and then as slowly sink
again. This may seem strange, at first sight, when it is remembered
that in reality the great sea-wave we are considering swept at the rate
of three or four hundred sea-miles an hour over the larger part of the
Pacific. But when the true character of ocean-waves is understood, when
it is remembered that there is no transference of the water itself at
this enormous rate, but simply a transmission of motion (precisely as
when in a high wind waves sweep rapidly over a corn-field, while yet
each cornstalk remains fixed in the ground), it will be seen that the
effects of the great sea-wave could only be perceived near the shore.
Even there, as we shall presently see, there was much to convey the
impression that the land itself was rising and falling rather than
that the deep was moved. But among the hundreds of ships which were
sailing upon the Pacific when its length and breadth were traversed by
the great sea-wave, there was not one in which any unusual motion was
perceived.
In somewhat less than three hours after the occurrence of the
earthquake, the ocean-wave inundated the port of Coquimbo, on the
Chilian seaboard, some 800 miles from Arica. An hour or so later it
had reached Constitucion, 450 miles farther south; and here for some
three hours the sea rose and fell with strange violence. Farther south,
along the shore of Chili, even to the island of Chiloe, the shore-wave
travelled, though with continually diminishing force, owing doubtless
to the resistance which the irregularities of the shore opposed to its
progress.
The northerly shore-wave seems to have been more considerable; and
a moment’s study of a chart of the two Americas will show that this
circumstance is highly significant. When we remember that the principal
effects of the land-shock were experienced within that angle which
the Peruvian Andes form with the long north-and-south line of the
Chilian and Bolivian Andes, we see at once that, had the centre of the
subterranean action been near the scene where the most destructive
effects were perceived, no sea-wave, or but a small one, could have
been sent towards the shores of North America. The projecting shores of
northern Peru and Ecuador could not have failed to divert the sea-wave
towards the west; and though a reflected wave might have reached
California, it would only have been after a considerable interval of
time, and with dimensions much less than those of the sea-wave which
travelled southwards. When we see that, on the contrary, a wave of even
greater proportions travelled towards the shores of North America,
we seem forced to the conclusion that the centre of the subterranean
action must have been so far to the west that the sea-wave generated by
it had a free course to the shores of California.
Be this as it may, there can be no doubt that the wave which swept
the shores of Southern California, rising upwards of sixty feet above
the ordinary sea-level, was absolutely the most imposing of all the
indirect effects of the great earthquake. When we consider that
even in San Pedro Bay, fully five thousand miles from the centre of
disturbance, a wave twice the height of an ordinary house rolled in
with unspeakable violence only a few hours after the occurrence of
the earth-throe, we are most strikingly impressed with the tremendous
energy of the earth’s movement.
Turning to the open ocean, let us track the great wave on its course
past the multitudinous islands which dot the surface of the great
Pacific.
The inhabitants of the Sandwich Islands, which lie about 6,300 miles
from Arica, might have imagined themselves safe from any effects
which could be produced by an earthquake taking place so far away
from them. But on the night between August 13 and 14, the sea around
this island-group rose in a surprising manner, insomuch that many
thought the islands were sinking and would shortly subside altogether
beneath the waves. Some of the smaller islands, indeed, were for a
time completely submerged. Before long, however, the sea fell again,
and as it did so the observers ‘found it impossible to resist the
impression that the islands were rising bodily out of the water.’ For
no less than three days this strange oscillation of the sea continued
to be experienced, the most remarkable ebbs and floods being noticed at
Honolulu, on the island of Woahoo.
But the sea-wave swept onwards far beyond these islands.
At Yokohama, in Japan, more than 10,500 miles from Arica, an enormous
wave poured in on August 14, but at what hour we have no satisfactory
record. So far as distance is concerned, this wave affords most
surprising evidence of the stupendous nature of the disturbance to
which the waters of the Pacific Ocean had been subjected. The whole
circumference of the earth is but 25,000 miles, so that this wave had
travelled over a distance considerably greater than two-fifths of the
earth’s circumference. A distance which the swiftest of our ships could
not traverse in less than five or six weeks had been swept over by this
enormous undulation in the course of a few hours.
More complete details reach us from the Southern Pacific.
Shortly before midnight the Marquesas Isles and the low-lying Tuamotu
group were visited by the great wave, and some of these islands were
completely submerged by it. The lonely Opara Isle, where the steamers
which run between Panama and New Zealand have their coaling station,
was visited at about half-past eleven in the evening by a billow which
swept away a portion of the coal depôt. Afterwards great waves came
rolling in at intervals of about twenty minutes, and several days
elapsed before the sea resumed its ordinary ebb and flow.
It was not until about half-past two on the morning of August 14, that
the Samoa Isles (sometimes called the Navigator Islands) were visited
by the great wave. The watchmen startled the inhabitants from their
sleep by the cry that the sea was about to overwhelm them; and already,
when the terrified people rushed from their houses, the sea was found
to have risen far above the highest watermark. But it presently began
to sink again, and then commenced a series of oscillations, which
lasted for several days and were of a very remarkable nature. Once in
every quarter of an hour the sea rose and fell, but it was noticed
that it rose twice as rapidly as it sank. This peculiarity is well
worth remarking. The eminent physicist Mallet speaks thus (I follow
Lyell’s quotation) about the waves which traverse an open sea: ‘The
great sea-wave, advancing at the rate of several miles in a minute,
consists, in the deep ocean, of a long low swell of enormous volume,
having an equal slope before and behind, and that so gentle that it
might pass under a ship without being noticed. But when it reaches the
edge of soundings its front slope becomes short and steep, while its
rear slope is long and gentle.’ On the shores visited by such a wave,
the sea would appear to rise more rapidly than it sank. We have seen
that this happened on the shores of the Samoan group, and therefore
the way in which the sea rose and fell on the days following the great
earthquake gave significant evidence of the nature of the sea-bottom in
the neighbourhood of these islands. As the change of the great wave’s
figure could not have been quickly communicated, we may conclude with
certainty that the Samoan Islands are the summits of lofty mountains,
whose sloping sides extend far towards the east.
This conclusion affords interesting evidence of the necessity of
observing even the seemingly trifling details of important phenomena.
The wave which visited the New Zealand Isles was altogether different
in character, affording a noteworthy illustration of another remark
of Mallet’s. He says that where the sea-bottom slopes in such a way
that there is water of some depth close in-shore, the great wave may
roll in and do little damage; and we have seen that so it happened in
the case of the Samoan Islands. But he adds, that ‘where the shore
is shelving, there will be first a retreat of the water, and then the
wave will break upon the beach and roll far in upon the land.’ This is
precisely what happened when the great wave reached the eastern shores
of New Zealand, which are known to shelve down to very shallow water,
continuing far away to sea towards the east:—
At about half-past three on the morning of August 14 the water began to
retreat in a singular manner from the Port of Lyttelton, on the eastern
shores of the southernmost of the New Zealand Islands. At length the
whole port was left entirely dry, and so remained for about twenty
minutes. Then the water was seen returning like a wall of foam ten
or twelve feet in height, which rushed with a tremendous noise upon
the port and town. Towards five o’clock the water again retired, very
slowly as before, not reaching its lowest ebb until six. An hour later,
a second huge wave inundated the port. Four times the sea retired and
returned with great power at intervals of about two hours. Afterwards
the oscillation of the water was less considerable, but it had not
wholly ceased until August 17, and only on the 18th did the regular ebb
and flow of the tide recommence.
Around the Samoan group the water rose and fell once in every fifteen
minutes, while on the shores of New Zealand each oscillation lasted
no less than two hours. Doubtless the different depths of water,
the irregular conformation of the island groups, and other like
circumstances, were principally concerned in producing these singular
variations. Yet they do not seem fully sufficient to account for so
wide a range of difference. Possibly a cause yet unnoticed may have
had something to do with the peculiarity. In waves of such enormous
extent, it would be quite impossible to determine whether the course of
the wave-motion was directed full upon a line of shore or more or less
obliquely. It is clear that in the former case the waves would seem to
follow each other more swiftly than in the latter, even though there
were no difference in their velocity.
Far on beyond the shores of New Zealand the great wave coursed,
reaching at length the coast of Australia. At dawn of August 14,
Moreton Bay was visited by five well-marked waves. At Newcastle, on
the Hunter River, the sea rose and fell several times in a remarkable
manner, the oscillatory motion commencing at half-past six in the
morning. But the most significant evidence of the extent to which
the sea-wave travelled in this direction was afforded at Port Fairy,
Belfast, South Victoria. Here the oscillation of the water was
distinctly perceived at midday on August 14; and yet, to reach this
point, the sea-wave must not only have travelled on a circuitous course
nearly equal in length to half the circumference of the earth, but
must have passed through Bass’s Straits, between Australia and Van
Diemen’s Land, and so have lost a considerable portion of its force and
dimensions.
When we remember that had not the effects of the earth-shock been
limited by the shores of South America, a wave of disturbance equal
in extent to that which travelled westward would have swept towards
the east, we see that the force of the shock was sufficient to have
disturbed the waters of an ocean covering the whole surface of the
earth. For the sea-waves which reached Yokohama in one direction and
Port Fairy in another had each traversed a distance nearly equal to
half the earth’s circumference; so that if the surface of the earth
were all sea, waves setting out in opposite directions from the centre
of disturbance would have met each other at the antipodes of their
starting-point.
It is impossible to contemplate the effects which followed the great
earthquake—the passage of a sea-wave of enormous volume over fully
one-third of the earth’s surface, and the force with which, at the
farthermost limits of its range, the wave rolled in upon shores more
than 10,000 miles from its starting-place—without feeling that those
geologists are right who deny that the subterranean forces of the
earth are diminishing in intensity. It may be difficult, perhaps,
to look on the effects which are ascribed to ancient earth-throes
without imagining for a while that the power of modern earthquakes is
altogether less. But when we consider fairly the share which time had
in those ancient processes of change, when we see that while mountain
ranges were being upheaved or valleys depressed to their present
position, race after race and type after type appeared on the earth,
and lived out the long lives which belong to races and to types, we
are recalled to the remembrance of the great work which the earth’s
subterranean forces are still engaged upon. Even now continents are
being slowly depressed or upheaved, even now mountain ranges are
being raised to a new level, table-lands are in process of formation,
and great valleys are being gradually scooped out. It may need an
occasional outburst such as the earthquake of August 1868 to remind
us that great forces are at work beneath the earth’s surface. But, in
reality, the signs of change have long been noted. Old shore-lines
shift their place, old soundings vary; the sea advances in one place
and retires in another; on every side Nature’s plastic hand is at work
modelling and remodelling the earth, in order that it may always be a
fit abode for those who are to dwell upon it.
(From _Fraser’s Magazine_, July 1870.)
_THE USEFULNESS OF EARTHQUAKES._
We have lately had fearful evidence of the energy of the earth’s
internal forces. A vibration which, when considered with reference
to the dimensions of the earth’s globe, may be spoken of as an
indefinitely minute quivering limited to an insignificant area, has
sufficed to destroy the cities and villages of whole provinces,
to cause the death of thousands of human beings, and to effect a
destruction of property which must be estimated by millions of pounds
sterling. Such a catastrophe as this serves indeed to show how poor
and weak a creature man is in presence of the grand workings of Nature.
The mere throes which accompany her unseen subterranean efforts suffice
to crumble man’s strongest buildings in a moment into dust, while the
unfortunate inhabitants are either crushed to death among the ruins,
or forced to remain shuddering spectators of the destruction of their
homes.
At first sight it may seem paradoxical to assert that earthquakes,
fearfully destructive as they have so often proved, are yet essentially
preservative and restorative phenomena; yet this is strictly the
case. Had no earthquakes taken place in old times, man would not now
be living on the face of the earth; if no earthquakes were to take
place in future, the term of man’s existence would be limited within
a range of time far less than that to which it seems likely, in all
probability, to be extended.
If the solid substance of the earth formed a perfect sphere in
ante-geologic times—that is, in ages preceding those to which our
present geologic studies extend—there can be no doubt that there was
then no visible land above the surface of the water; the ocean must
have formed a uniformly deep covering to the submerged surface of
the solid globe. In this state of things, nothing but the earth’s
subterranean forces could tend to the production of continents and
islands. Let me be understood. I am not referring to the possibility or
impossibility that lands and seas should suddenly have assumed their
present figure without convulsion of any sort; this _might_ have
happened, since the Creator of all things can doubtless modify all
things according to His will; I merely say that, assuming that in the
beginning, as now, He permitted all things to work according to the
laws He has appointed, then, undoubtedly, the submerged earth must have
risen above the sea by the action of those very forms of force which
produce the earthquake in our own times.
However this may be, it is quite certain that when once continents
and lands had been formed, there immediately began a struggle between
destructive and restorative (rather, perhaps, than preservative) forces.
The great enemy of the land is water, and water works the destruction
of the land in two principal ways.
In the first place the sea tends to destroy the land by beating on its
shores, and thus continually washing it away. It may seem at first
sight that this process must necessarily be a slow one; in fact, many
may be disposed to say that it is certainly a slow process, since
we see that it does not alter the forms of continents and islands
perceptibly in long intervals of time. But, as a matter of fact, we
have never had an opportunity of estimating the full effects of this
cause, since its action is continually being checked by the restorative
forces we shall presently have to consider. Were it not thus checked,
there can be little doubt that its effects would be cumulative; for the
longer the process continued—that is, the more the land was beaten
away—the higher would the sea rise, and the greater power would it have
to effect the destruction of the remaining land.
I proceed to give a few instances of the sea’s power of effecting
the rapid destruction of the land when nothing happens to interfere
with the local action—premising, that this effect is altogether
insignificant in comparison with that which would take place, even
in that particular spot, if the sea’s action were _everywhere_ left
unchecked.
The Shetland Isles are composed of substances which seem, of all
others, best fitted to resist the disintegrating forces of the
sea—namely, granite, gneiss, mica-slate, serpentine, greenstone, and
many other forms of rock: yet, exposed as these islands are to the
uncontrolled violence of the Atlantic Ocean, they are undergoing
a process of destruction which, even within historical times, has
produced very noteworthy changes. ‘Steep cliffs are hollowed out,’
says Sir Charles Lyell, ‘into deep caves and lofty arches; and almost
every promontory ends in a cluster of rocks imitating the forms of
columns, pinnacles, and obelisks.’ Speaking of one of the islands of
this group, Dr. Hibbert says: ‘The isle of Stennes presents a scene
of unequalled desolation. In stormy winters, large blocks of stone
are overturned, or are removed from their native beds, and hurried to
a distance almost incredible. In the winter of 1802, a tabular mass,
eight feet two inches by seven feet, and five feet one inch thick, was
dislodged from its bed, and carried to a distance of from eighty to
ninety feet. In other parts of the Shetland Isles, where the sea has
encountered less solid materials, the work of destruction has proceeded
yet more effectively. In Roeness, for example, the sea has wrought
its way so fiercely that a large cavernous aperture 250 feet long has
been hollowed out. But the most sublime scene,’ says Dr. Hibbert, ‘is
where a mural pile of porphyry, escaping the process of disintegration
that is devastating the coast, appears to have been left as a sort of
rampart against the inroads of the ocean. The Atlantic, when provoked
by wintry gales, batters against it with all the force of real
artillery; and the waves, in their repeated assaults, have at length
forced for themselves an entrance. This breach, named the Grind of the
Navir, is widened every winter by the overwhelming surge that, finding
a passage through it, separates large stones from its sides, and forces
them to a distance of no less than 180 feet. In two or three spots,
the fragments which have been detached are brought together in immense
heaps, that appear as an accumulation of cubical masses, the product of
some quarry.’
Let us next turn to a portion of the coast-line of Great Britain
which is neither defended, on the one hand, by barriers of rock, nor
attacked, on the other, by the full fury of the Atlantic currents.
Along the whole coast of Yorkshire we find evidences of a continual
process of dilapidation. Between the projecting headland of
Flamborough and Spurn Point (the coast of Holderness) the waste is
particularly rapid. Many spots, which are now mere sandbanks, are
marked in the old maps of Yorkshire as the sites of ancient towns and
villages. Speaking of Hyde (one of these), Pennant says: ‘Only the
tradition is left of this town.’ Owthorne and its church have been for
the most part destroyed, as also Auburn, Hartburn, and Kilnsea. Mr.
Phillips, in his ‘Geology of Yorkshire,’ states that not unreasonable
fears are entertained that, at some future time, Spurn Point itself
will become an island, or be wholly washed away, and then the ocean,
entering into the estuary of the Humber, will cause great devastation.
Pennant states that ‘several places, once towns of note upon the
Humber, are now only recorded in history; and Ravensperg was at one
time a rival of Hull, and a port so very considerable in 1332, that
Edward Baliol and the confederate English barons sailed from hence to
invade Scotland; and Henry IV., in 1399, made choice of this port to
land at, to effect the deposal of Richard II.; yet the whole of this
has since been devoured by the merciless ocean; extensive sands, dry
at low water, are to be seen in their stead.’ The same writer also
describes Spurn Point as shaped like a sickle, and the land to the
north, he says, was ‘perpetually preyed on by the fury of the German
Sea, which devours whole acres at a time.’
The decay of the shores of Norfolk and Suffolk is also remarkably
rapid. Sir Charles Lyell relates some facts which throw an interesting
light on the ravages which the sea commits upon the land here. It was
computed that when a certain inn was built at Sherringham, seventy
years would pass before the sea could reach the spot: ‘the mean loss
of land being calculated from previous observations to be somewhat
less than one yard annually.’ But no allowance had been made for the
fact that the ground sloped _from_ the sea. In consequence of this
peculiarity, the waste became greater and greater every year as the
cliff grew lower. ‘Between the years 1824 and 1829, no less than
seventeen yards were swept away;’ and when Sir Charles Lyell saw the
place, only a small garden was left between the building and the
sea. I need hardly add that all vestiges of the inn have long since
disappeared. Lyell also relates that, in 1829, there was a depth of
water sufficient to float a frigate at a point where, less than half a
century before, there stood a cliff fifty feet high with houses upon it.
I have selected these portions of the coast of Great Britain, not
because the destruction of our shores is greater here than elsewhere,
but as serving to illustrate processes of waste and demolition which
are going on around all the shores, not merely of Great Britain, but
of every country on the face of the earth. Here and there, as I have
said, there are instances in which a contrary process seems to be in
action. Low-lying banks and shoals are formed—sometimes along stretches
of coast extending for a considerable distance. But when we consider
these formations closely, we find that they rather afford evidence of
the energy of the destructive forces to which the land is subject
than promise to make up for the land which has been swept away. In the
first place, every part of these banks consists of the debris of other
coasts. Now we cannot doubt that of earth which is washed away from
our shores, by far the larger part finds its way to the bottom of the
deep seas; a small proportion only can be brought (by some peculiarity
in the distribution of ocean-currents, or in the progress of the
tidal wave) to aid in the formation of shoals and banks. The larger,
therefore, such shoals and banks may be, the larger must be the amount
of land which has been washed away never to reappear. And although
banks and shoals of this sort grow year by year larger and larger, yet
(unless added to artificially) they continue always either beneath
the surface of the water in the case of shoals, or but very slightly
raised above the surface. Now, if we suppose the destruction of land
to proceed unchecked, it is manifest that at some period, however
remote, the formation of shoals and banks must come to an end, owing
to the continual diminution of the land from the demolition of which
they derive their substance. In the meantime, the bed of the sea would
be continually filling up, the level of the sea would be continually
rising, and thus the banks would be either wholly submerged through the
effect of this cause alone, or they would have so slight an elevation
above the sea-level that they would offer little resistance to the
destructive effects of the sea, which would then have no other land to
act upon.
But we have yet to consider the second principal cause of the wasting
away of the land. The cause we have just been dealing with acts upon
the shores or outlines of islands and continents; the one we have now
to consider acts upon their interior. Many, perhaps, would hardly
suppose that the fall of rain upon the land could have any appreciable
influence in the demolition of continents; but, as a matter of fact,
there are few causes to which geologists attribute more importance.
The very fact that enormous deltas have been formed at the mouths of
many rivers—in other words, the actual growth of continents through
the effects of rainfall—is a proof how largely this cause must tend
to destroy and disintegrate the interiors of our continents. Dwelling
on this point, Sir Charles Lyell presents the following remarkable
illustration: ‘During a tour in Spain,’ he writes, ‘I was surprised to
see a district of gently undulating ground in Catalonia, consisting of
red and grey sandstone, and in some parts of red marl, almost entirely
denuded of herbage; while the roots of the pines, holm oaks, and some
other trees, were half exposed, as if the soil had been washed away
by a flood. Such is the state of the forests, for example, between
Oristo and Vich, and near San Lorenzo. But being overtaken by a violent
thunderstorm in the month of August, I saw the whole surface, even
the highest levels of some flat-topped hills, streaming with mud,
while on every declivity the devastation of torrents was terrific.
The peculiarities in the physiognomy of the district were at once
explained; and I was taught that, in speculating on the greater effects
which the direct action of rain may once have produced on the surface
of certain parts of England, we need not revert to periods when the
heat of the climate was tropical.’
Combining the effects of the sea’s action upon the shores of
continents, and of the action of rain upon their interior, and
remembering that unless the process of demolition were checked in some
way, each cause would act from year to year with new force—one through
the effects of the gradual rise of the sea-bed, and the other through
the effects of the gradual increase of the surface of ocean exposed
to the vaporising action of the sun, which increase would necessarily
increase the quantity of rain yearly precipitated on the land—we see
the justice of the opinion expressed by Sir John Herschel, that, ‘had
the primeval world been constructed as it now exists, time enough has
elapsed, and force enough directed to that end has been in activity,
_to have long ago destroyed every vestige of land_.’
We see, then, the necessity that exists for the action of some
restorative or preservative force sufficient to counteract the effects
of the continuous processes of destruction indicated above. If we
consider, we shall see that the destructive forces owe their efficiency
to their levelling action, that is, to their influence in reducing the
solid part of the earth to the figure of a perfect sphere; therefore
the form of force which is required to counteract them is one that
shall tend to produce irregularities in the surface-contour of the
earth. And it will be remarked, that although _upheaval_ is the
process which appears at first sight to be the only effectual remedy
to the levelling action of rains and ocean-currents, yet the forcible
depression of the earth’s surface may prove in many instances yet more
effective, since it may serve to reduce the sea-level in other places.
Now, the earth’s subterranean forces serve to produce the very effects
which are required in order to counteract the continual disintegration
of the shores and interior parts of continents. In the first place,
their action is not distributed with any approach to uniformity over
different parts of the earth’s crust, and therefore the figure they
tend to give to the surface of that crust is not that of a perfect
sphere. This, of itself, secures the uprising of some parts of the
solid earth above the sea-level. But this is not all. On a comparison
of the various effects due to the action of subterranean forces, it
has been found that the forces of upheaval act (on the whole) more
powerfully under continents, and especially under the shore-lines of
continents, while the forces of depression act most powerfully (on the
whole) under the bed of the ocean. It need hardly be said that whenever
the earth is upheaved in one part, it must be depressed somewhere else.
Not necessarily at the same instant, it should be remarked. The process
of upheaval may be either momentarily accompanied by a corresponding
process of depression, or the latter process may take place by a
gradual action of the elastic powers of the earth’s crust; but, in one
way or the other, the balance between upheaval and depression must be
restored. Hence, if it can be shown that for the most part the forces
of upheaval act underneath the land, it follows—though we may not be
able to recognise the fact by obvious visible signs—that processes of
depression are taking place underneath the ocean. Now, active volcanoes
mark the centre of a district of upheaval, and most volcanoes are near
the sea, as if (though, of course, this is not the true explanation)
Nature had provided against the inroads of the ocean by seating the
earth’s upheaving forces just where they are most wanted.
Even in earthquake districts which have no active vent, the same law is
found to prevail. It is supposed by the most eminent seismologists that
earthquake regions around a volcano, and earthquake regions apparently
disconnected from any outlet, differ only in this respect, that in the
one case the subterranean forces have had sufficient power to produce
the phenomena of eruption, while in the other they have not. ‘In
earthquakes,’ says Humboldt, ‘we have evidence of a volcano-producing
force; but such a force, as universally diffused as the internal heat
of the globe, and proclaiming itself everywhere, rarely acts with
sufficient energy to produce actual eruptive phenomena; and when it
does so, it is only in isolated and particular places.’
Of the influence of the earth’s subterranean forces in altering
the level of land, I might quote many remarkable instances, but
considerations of space compel me to confine myself to two or three.
The slow processes of upheaval or depression may, perhaps, seem less
immediately referable to subterranean action than those which are
produced during the progress of an actual earthquake. I pass over,
therefore, such phenomena as the gradual uprising of Sweden, the slow
sinking of Greenland, and (still proceeding westward) the gradual
uprising of Nova Scotia and the shores of Hudson’s Bay. Remarkable
and suggestive as these phenomena really are, and indisputable as the
evidence is on which they rest, they will probably seem much less
striking to the reader than those which I am now about to quote.
On the 19th of November, 1822, a widely felt and destructive earthquake
was experienced in Chili. On the next day, it was noticed for the first
time that a broad line of sea-coast had been deserted by the sea for
more than one hundred miles. A large part of this tract was covered by
shell-fish, which soon died, and exhaled the most offensive effluvia.
Between the old low-water mark and the new one, the fishermen found
burrowing shells, which they had formerly had to search for amidst the
surf. Rocks some way out to sea which had formerly been covered, were
now dry at half ebb-tide.
Careful measurements showed that the rise of the land was greater at
some distance inshore than along the beach. The watercourse of a mill
about a mile inland from the sea had gained a fall of fourteen inches
in little more than a hundred yards. At Valparaiso, the rise was three
feet; at Quintero, four feet.
In February 1835, and in November 1837, a large tract of Chili was
similarly shaken, a permanent rise of two feet following the former
earthquake, and a rise of eight feet the latter.
The earthquake which took place at Cutch in 1819 is perhaps in
some respects yet more remarkable. In this instance, phenomena of
subsidence, as well as phenomena of upheaval, were witnessed. The
estuary of the Indus, which had long been closed to navigation—being,
in fact, only a foot deep at ebb-tide, and never more than six feet at
flood—was deepened in parts to more than eighteen feet at low water.
The fort and village of Sindree were submerged, only the tops of houses
and walls being visible above the water. But although this earthquake
seemed thus to have a land-destroying, instead of a land-creating
effect, yet the instances of upheaval were, even in this case, far more
remarkable than those of depression. ‘Immediately after the shock,’
says Sir Charles Lyell, ‘the inhabitants of Sindree saw at a distance
of five miles and a half from their village a long elevated mound,
where previously there had been a low and perfectly level plain. To
this uplifted tract they gave the name of Ulla-Bund, or the “Mound of
God,” to distinguish it from several artificial dams previously thrown
across the eastern arm of the Indus. It has been ascertained,’ he adds,
‘that this new-raised country is upwards of fifty miles in length from
east to west, running parallel to the line of subsidence which caused
the grounds around Sindree to be flooded. The breadth of the elevation
is conjectured to be in some parts sixteen miles, and its greatest
ascertained height above the original level of the delta is ten feet—an
elevation which appears to the eye to be very uniform throughout.‘
(From _Chambers’s Journal_, November 7, 1868.)
_THE FORCING POWER OF RAIN._
There is an old proverb which implies that England need never fear
drought; and we have had clear evidence this year (1868) that an
exceptionally dry summer is not necessarily followed by a bad harvest.
But I believe that when a balance is carefully struck between the good
and the evil effects resulting from excessive drought in England, it
will be found that the latter largely prevail. In fact, it is only
necessary to observe the effects which have followed the recent wet
weather to recognise the fact that rain has a forcing power, the
very diminished supply of which at the due season cannot fail to
have seriously injurious effects. In various parts of England we see
evidences of the action of such a power during the present autumn in
the blossoming of trees, in the flowering of primroses and other spring
plants, in rich growths of fungi, and in various other ways. It cannot
be doubted that there is here a comparative waste of powers which,
expended in due season, would have produced valuable results.
The modern theories of the correlation of force suffice to show how
enormous a loss a country suffers when there is a failure in the
supply of rain, or when that supply comes out of its due season. When
we consider rain in connection with the causes to which it is due, we
begin to recognise the enormous amount of power of which the ordinary
rainfall of a country is the representative; and we can well understand
how it is that ‘the clouds drop fatness on the earth.’
The sun’s heat is, of course, the main agent—we may almost say the
only agent—in supplying the rainfall of a country. The process of
evaporation carried on over large portions of the ocean’s surface
is continually storing up enormous masses of water in the form of
invisible aqueous vapour, ready to be transformed into cloud, then
wafted for hundreds of miles across seas and continents, to be finally
precipitated over this or that country, according to the conditions
which determine the downfall of rain. These processes do not appear, at
first sight, indicative of any very great expenditure of force, yet in
reality the force-equivalent of the rain-supply of England alone for a
single year is something positively startling. It has been calculated
that the amount of heat required to evaporate a quantity of water which
would cover an area of 100 square miles to a depth of one inch would
be equal to the heat which would be produced by the combustion of half
a million tons of coals. The amount of force of which this consumption
of heat would be the equivalent corresponds to that which would be
required to raise a weight of upwards of one thousand millions of tons
to a height of one mile. Now, when we remember that the area of Great
Britain and Ireland is about 120,000 square miles, and that the annual
rainfall averages about 25 inches, we see that the force-equivalent
of the rainfall is enormous. All the coal which could be raised from
our English coal mines in hundreds of years would not give out heat
enough to produce England’s rain-supply for a single year. When to this
consideration we add the circumstance that the force of rain produces
bad as well as good effects—the former when the rain falls at undue
seasons or in an irregular manner, the latter only when the rainfall
is distributed in the usual manner among the seasons—we see that an
important loss accrues to a country in such exceptional years as the
present.
There are few subjects more interesting than those depending on the
correlation of physical forces; and we may add that there are few the
study of which bears more largely on questions of agricultural and
commercial economy. It is only of late years that the silent forces
of nature—forces continually in action, but which are too apt to pass
unnoticed and unrecognised—have taken their due place in scientific
inquiry. Strangely enough, the subject has been found to have at once
a most practical bearing on business relations, and an aspect more
strikingly poetical than any other subject, perhaps, which men of
science have ever taken in hand to investigate. We see the ordinary
processes of Nature, as they are termed, taking their place in the
workshop of modern wealth, and at the same time exhibited in a hundred
striking and interesting physical relations. What, for instance, can
be stranger or more poetical than the contrast which Professor Tyndall
has instituted between that old friend of the agriculturist—the
wintry snow-flake—and the wild scenery of the Alps? ‘I have seen,’ he
says, ‘the wild stone-avalanches of the Alps, which smoke and thunder
down the declivities with a vehemence almost sufficient to stun the
observer. I have also seen snow-flakes descending so softly as not to
hurt the fragile spangles of which they were composed; yet to produce
from aqueous vapour a quantity which a child could carry of that tender
material demands an exertion of energy competent to gather up the
shattered blocks of the largest stone-avalanche I have ever seen, and
pitch them to twice the height from which they fell.’
I may point out in this place the important connection which exists
between the rainfall of a country and the amount of forest land. I
notice that in parts of America attention is being paid—with markedly
good results—to the influence of forests in encouraging rainfall. We
have here an instance in which cause and effect are interchangeable.
Rain encourages the growth of an abundant vegetation, and abundant
vegetation in turn tends to produce a state of the superincumbent
atmosphere which encourages the precipitation of rain. The consequence
is, that it is very necessary to check, before it is too late, the
processes which lead to the gradual destruction of forests. If these
processes are continued until the climate has become excessively dry,
it is almost impossible to remedy the mischief, simply because the
want of moisture is destructive to the trees which may be planted to
encourage rainfall. Thus there are few processes more difficult (as
has been found by experience in parts of Spain and elsewhere) than
the change of an arid region into a vegetation-covered district. In
fact, if the region is one of great extent, the attempt to effect
such a change is a perfectly hopeless one. On the other hand, the
contrary process—that is, the attempt to change a climate which is too
moist into one of less humidity—is in general not attended with much
difficulty. A judicious system of clearing nearly always leads to the
desired result.
The dryness of the past year has not been due to the want of moisture
in the air, nor to the exceptionally unclouded condition of our
skies. I believe that, on the whole, the skies have been rather more
cloudy than usual this year. The fact that so little dew has fallen
is a sufficient proof that the nights have been on the whole more
cloudy than usual, since, as is well known, the presence of clouds,
by checking the radiation of the earth’s heat, prevents (or at least
diminishes) the formation of dew. The fact would seem to be that the
westerly and south-westerly winds which usually blow over England
during a considerable part of the year, bringing with them large
quantities of aqueous vapour from above the great Gulf Stream, have
this year blown somewhat higher than usual. Why this should be it
is not very easy to say. The height of the vapour-laden winds is
usually supposed to depend on the heat of the weather. In summer, for
instance, the clouds range higher, and therefore travel farther inland
before they fall in rain. In winter, on the contrary, they travel
low, and hence the rain falls more freely in the western than in the
eastern counties during winter. A similar relation prevails in the
Scandinavian peninsula—Norway receiving more rain in winter than in
summer, while Sweden receives more rain in summer than in winter. But
this summer the rain-clouds have blown so much higher than usual as to
pass beyond England altogether. Possibly we may find an explanation
in the fact that before reaching our shores at all the clouds were
relieved by heavy rainfalls—probably due to some exceptional electrical
relations—over parts of the Atlantic Ocean. It is stated that the
steam-ships from America this summer were, in many instances, drenched
by heavy showers until they neared the coasts of England.
(From the _Daily News_, October 5, 1868.)
_A SHOWER OF SNOW-CRYSTALS._
Yesterday morning a remarkably fine fall of snow-stars took place over
many parts of London. The crystals were larger and more perfectly
formed than is commonly the case in our latitudes, where the conditions
requisite for the formation of these beautiful objects are less
perfectly fulfilled than in more northerly regions. Many forms were to
be noticed which the researches of Scoresby, Glaisher, and Lowe have
shown to be somewhat uncommon.
Some of my readers will perhaps be surprised to learn that no less
than 1,000 different kinds of snow-crystals have been noticed by the
observers named above, and that a large proportion of them have been
figured and described. The patterns are of wonderful beauty. A strange
circumstance connected with these objects is the fact that for the most
part they are found, on a close examination, to be formed of minute
coloured crystals—some red, some green, others blue or purple. In fact,
all the colours of the rainbow are to be seen in the delicate tracery
of these fine hexagonal stars. So that in the perfect whiteness of the
driven snow we have an illustration of the well-known fact that the
colours of the rainbow combine to form the purest white. For the common
snow-flake is formed of a large number of such tiny crystals as were
falling yesterday; though their beauty is destroyed in the snow-flake,
through the effects of collision and partial melting. It may not
be very commonly known that ordinary ice, also, is composed of a
combination of crystals presenting all the regularity of formation seen
in the snow-crystals. This would scarcely be believed by anyone who
examined a rough mass of ice taken from the surface of a frozen lake.
Yet, if a slice be cut from the mass and placed in the sun’s light, or
before a fire, the beautiful phenomena called ice-flowers make their
appearance. ‘A fairy seems to have breathed upon the ice, and caused
transparent flowers of exquisite beauty suddenly to blossom in myriads
within it.’
When we remember that the enormous icebergs of the Arctic and Antarctic
seas, the snow-caps which crown the Alps and Andes and Himalayas,
and the glaciers which urge their way with resistless force down the
mountain valleys, are all made up of these delicate and beautiful
snow-flowers, we are struck with the force of the strange contrasts
which Nature presents to our contemplation. We may say of the
snow-crystals what Tennyson said of the small sea-shell. Each snow-star
is
Frail, but a work divine
Made so fairily well,
So exquisitely minute,
A miracle of design.
Yet—massed together with all the prodigality of Nature’s unsparing
hand—they crown the everlasting hills; or, falling in avalanche and
glacier, overwhelm the stoutest works of man; or, in vast islands of
floating ice, show themselves to be
Of force to withstand, year upon year, the shock
Of cataract seas that snap the three-decker’s oaken spine.
(From the _Daily News_, March 11, 1869.)
_LONG SHOTS._
Our artillerists have paid more attention of late years to the
destructive properties of various forms of cannon than to the question
of range. It was different when first the rifling of cannon was under
discussion. Then the subject which was most attentively considered
(after accuracy of fire) was the range which might possibly be
attained by various improvements in the structure of rifled cannon.
Many of my readers will remember how, soon after the construction
of Armstrong guns had been commenced in the Government factories, a
story was spread abroad of the wonderful practice which had been made
with this gun at a range of seven miles. At that tremendous range, a
shot had been fired into the middle of a flock of geese, according
to one version of the story; but this was presently improved upon,
and we were told that a bird had been singled out of the flock by the
artillerists and successfully ‘potted.’ Many believed this little
narrative; though some few, influenced perhaps by the consideration
that a flock of geese would not be visible at a distance of seven
miles, were obstinately incredulous. Presently it turned out that the
Armstrong gun was incapable of throwing a shot to a distance of seven
miles; so that a certain air of improbability has since attached to
the narrative. Still there were not wanting those who referred to
‘Queen Anne’s pocket-pistol’—the cannon which was able to throw shot
across the Straits of Dover; and in the fulness of their faith in that
mythical piece of ordnance, they refused to believe that the skill of
modern artillerists was unequal to the construction of cannon even more
effective.
If there are any who still believe in the powers ascribed to the
far-famed ‘pocket-pistol,’ they will find their confidence in modern
artillery largely shaken by the announcement that it is considered a
great matter that one of Whitworth’s cannon should have thrown a shot
to a distance of very nearly six miles and a half. Not only is this
so, however, but it is well known that no piece of ordnance has ever
flung a projectile to so great a distance since first fire-arms were
invented; and it may be safely predicted that men will never be able
to construct a cannon which—as far as range is concerned—will do much
better than this one of Mr. Whitworth’s. The greatest range which
had ever before been attained fell somewhat short of six miles. The
7-inch steel gun contrived by Mr. Lynall Thomas had flung a projectile
weighing 175 lbs. to a distance of 10,075 yards; and, according to
General Lefroy’s ‘Handbook of Artillery,’ that was the greatest range
ever recorded. But Mr. Whitworth’s cannon has thrown a shot more than
1,000 yards farther.
Very few have any idea of the difficulties which oppose themselves to
the attainment of a great range in artillery practice. It may seem,
at first sight, the simplest possible matter to obtain an increase of
range. Let the gun be made but strong enough to bear a sufficient
charge, and range seems to be merely a question of the quantity of
powder made use of. But in reality the matter is much more complicated.
The artillerist has to contrive that the whole of the powder made use
of shall be burned before the shot leaves the cannon, and yet that the
charge shall not explode so rapidly as to burst the cannon. If he used
some forms of powder, very useful for special purposes, half the charge
would be blown out without doing its share of work. On the other hand,
there are some combustibles (as gun-cotton and the nitrates) which burn
so fast that the gun would be likely to burst before the shot could be
expelled. Then, again, the shot must fit so closely that there shall
be no windage, and yet not so closely as to resist too much the action
of the exploding powder. Again, there is the form of the shot to be
considered. A sphere is not the solid which passes most readily through
a resisting medium like the air; and yet, other projectiles, which are
best so long as they maintain a certain position, meet with a greater
resistance when once they begin to move unsteadily. The conoid used in
ordinary rifle practice, for example, passes much more freely through
the air, point first, than an ordinary spherical bullet; but if the
point did not travel first, as would happen but for the rifling, or
even if the conoidal bullet ‘swayed about’ on its course, it would meet
with more resistance than a spherical bullet. Hence the question of
‘fast or slow rifling’ has to be considered. ‘Fast rifling’ gives the
greater spin, but causes more resistance to the exit of the shot from
the barrel; with ‘slow-rifling,’ these conditions are reversed.
And then the common notion is that a cannon-ball travels in the curve
called a parabola, and that artillerists have nothing to do but to
calculate all about this parabola, and to deduce the range from
the initial velocity according to some simple principles depending
on the properties of the curve. All this is founded on a complete
misapprehension of the true difficulties in the way of the problem.
Only projectiles thrown with small velocity from the earth travel in
parabolic paths. A cannon-ball follows a wholly different kind of
curve. The resistance of the air, which seems to most persons a wholly
insignificant item in the inquiry, is so enormous in the case of a
cannon-ball as to become by far the most important difficulty in the
way of the practical artillerist. When a 250-lb. shot is hurled with
such force from a gun as to cover a range of six miles, the resistance
of the air is about forty times the weight of the ball—that is, is
equivalent to a weight of upwards of four tons. The range in such a
case as this is but a small fraction of that which would be given by
the ordinary parabolic theory.
As regards artillery practice in war, there are other difficulties in
the attainment of a very extended range. Cannon meant for battering
down forts cannot possibly be used in the same way that Whitworth’s was
used at Shoeburyness. If the shot flung from this gun at an elevation
of thirty-three degrees could have been watched, it would have been
found that it fell to the earth at a much greater angle—that is, much
more nearly in a perpendicular direction. On the ordinary parabolic
theory, of course, the angle of fall would be the same as the angle
of elevation, but under actual circumstances there is an important
difference. If forts are to be battered down, however, it will not
serve that they should be struck from above; our artillerists must
perforce keep to the old method of pounding away at the face of the
forts they attack. Therefore, an elevation which is all very well for
mortars—that is, when the question merely is of flinging a bomb into a
town or fortress—is utterly unsuited for ordinary artillery. With an
elevation of ten degrees, Whitworth’s cannon scarcely projected the
250-lb. shot to a distance of three miles.
The progress of the modern science of gunnery certainly tends to
increase the distance at which armies will engage each other. With
field artillery flinging shot to a distance of two or three miles,
and riflemen able to make tolerably sure practice at a distance
of three-quarters of a mile, we are not likely often to hear of
hand-to-hand conflicts in future warfare. The use of breech-loaders
will also tend to the same effect. Hitherto we have scarcely had
experience of the results which these changes are to produce on modern
warfare. At Sadowa breech-loaders did not encounter breech-loaders,
and it was easy for the victors in that battle to come to close
quarters with their enemies. But in a battle where both sides are armed
with breech-loaders, we shall probably see another sort of affair
altogether. The bayonet will be an almost useless addition to the
soldier’s arms; a charge of cavalry upon well-armed infantry will be
almost as hopeless as the famous Balaclava charge; and the artillery
on either side will have to play a game at long bowls. I venture to
anticipate that the first great European war will introduce a total
change into the whole system of warlike manœuvres.[14]
(From the _Daily News_, November 1868.)
_INFLUENCE OF MARRIAGE ON THE DEATH-RATE._
The Royal Commission on the Law of Marriage has attracted attention
to many singular and instructive results of modern statistical
inquiry. Not the least important of these is the apparent influence
of marriage on the death-rate. For several years it has been noticed
by statisticians that the death-rate of unmarried men is considerably
higher than the death-rate of married men and widowers. I believe that
Dr. Stark, Registrar-General for Scotland, was one of the first to
call attention to this peculiarity, as evidenced by the results of two
years’ returns for Scotland. But the law has since been confirmed by a
far wider range of statistical inquiry. The relative proportion between
the death-rates of the married and of the unmarried is not absolutely
uniform in different countries, but it is fairly enough represented
by the following table, which exhibits the mortality per thousand of
married and unmarried men in Scotland:—
Ages. Husbands and Widowers. Unmarried.
20 to 25 6·26 12·31
25 to 80 8·23 14·94
30 to 35 8·65 15·94
35 to 40 11·67 16·02
40 to 45 14·07 18·35
45 to 50 17·04 21·18
50 to 55 19·54 26·34
55 to 60 26·14 28·54
60 to 65 35·63 44·54
65 to 70 52·93 60·21
70 to 75 81·56 102·71
75 to 80 117·85 143·94
80 to 85 173·88 195·40
From this table we are to understand that out of one hundred thousand
married persons (including widowers) from 20 to 25 years old, 626 die
in the course of each year; while out of a similar number of unmarried
persons, between the same ages, no less than 1,231 die in each
year. And in like manner all the other lines of the table are to be
interpreted.
Commenting on the evidence supplied by the above figures, Dr.
Stark stated that ‘bachelorhood is more destructive to life than
the most unwholesome trades, or than residence in an unwholesome
house or district, where there has never been the most distant
attempt at sanitary improvement of any kind.’ And this view has been
very generally accepted, not only by the public, but by professed
statisticians. Yet, as a matter of fact, I believe that no such
inferences can legitimately be drawn from the above table. Dr. Stark
appears to me to have fallen into the mistake, which M. Quetelet tells
us is so common, of trying to make his statistics carry more weight
than they are capable of bearing. It is important that the matter
should be put in a just light, for the Royal Commission on the Law of
Marriage has revealed no more striking fact than that of the prevalence
of immature marriages, and such reasoning as Dr. Stark’s certainly
cannot tend to discourage these unwise alliances. If death strikes down
in five years only half as many of those who are married as of those
who are unmarried between the ages of 20 and 25 (as appears from the
above table), and if the proportion of deaths between the two classes
goes on continually diminishing in each successive lustre (as is also
shown by the above table), it seems reasonable to infer that the
death-rate would be even more strikingly disproportionate for persons
between the ages of fifteen and twenty than for persons between the
ages of twenty and twenty-five. I believe, indeed, that if Dr. Stark
had extended his table to include the former ages, the result would
have been such as I have indicated. Yet few will suppose that very
youthful marriages can exercise so singularly beneficial an effect.
To many Dr. Stark’s conclusion may appear to be a natural and obvious
_sequitur_ from the evidence upon which it is founded. Admitting the
facts—and I see no reason for doubting them—it may appear at first
sight that we are bound to accept the conclusion that matrimony is
favourable to longevity. Yet the consideration of a few parallel cases
will suffice to show how small a foundation the figures I have quoted
supply for such a conclusion. What would be thought, for example,
of any of the following inferences?—Among hot-house plants there is
observed a greater variety and brilliance of colour than among those
which are kept in the open air; therefore the housing of plants
conduces to the splendour of their colouring. Or again: The average
height of Life Guardsmen is greater than that of the rest of the male
population; therefore to be a Life Guardsman conduces to tallness of
stature. Or to take an example still more closely illustrative of Dr.
Stark’s reasoning: The average longevity of noblemen exceeds that of
untitled persons; therefore to have a title is conducive to longevity;
or borrowing his words, to remain without a title ‘is more destructive
to life than the most unwholesome trades, or than residence in an
unwholesome house or district, where there has never been the most
distant attempt at sanitary improvement of any kind.’
We know that the inference is absurd in each of the above instances,
and we are able at once to show where the flaw in the reasoning lies.
We know that splendid flowers are more commonly selected for housing,
and that Life Guardsmen are chosen for their tallness, so that we are
prevented from falling into the mistake of ascribing splendour of
colour in the one instance, or tallness in the other, to the influence
of causes which have nothing whatever to do with those attributes;
nor is anyone likely to ascribe the longevity of our nobility to the
possession of a title. Yet there is nothing in any one of the above
inferences which is in reality more unsound than Dr. Stark’s inference
from the mortality bills, when the latter are considered with due
reference to the principles of interpretation which statisticians are
bound to follow.
The fact is, that in dealing with statistics the utmost care is
required in order that our inferences may not be pushed beyond the
evidence afforded by our facts. In the present instance, we have
simply to deal with the fact that the death-rate of unmarried men is
higher than the death-rate of married men and widowers. From this
fact we cannot reason as Dr. Stark has done to a _simple_ conclusion.
All that we can do is to show that one of _three_ conclusions must be
adopted:—Either matrimony is favourable (directly or indirectly) to
longevity, in a degree sufficient wholly to account for the observed
peculiarity; or a principle of selection—the effect of which is
such as, on the whole, to fill the ranks of married men from among
the healthier and stronger portion of the community—operates in a
sufficient degree to account wholly for the observed death-rates; or
lastly, the observed death-rates are due to the combination, in some
unknown proportion, of the two causes just mentioned.
No reasonable doubt can exist, as it seems to me, that the third is the
true conclusion to be drawn from the evidence supplied by the mortality
bills. Unfortunately, the conclusion thus deduced is almost valueless,
because we are left wholly in doubt as to the proportion which subsists
between the effects to be ascribed to the two causes thus shown to be
in operation.
It scarcely required the evidence of statistics to prove that each
cause must operate to some extent.
It is perfectly obvious, on the one hand, that although hundreds of
men who would be held by insurance companies to be ‘bad lives’ may
contract marriage, yet on the whole a principle of selection is in
operation which must tend to bring the healthier portion of the male
community into the ranks of the married, and to leave the unhealthier
in the state of bachelorhood. A little consideration will show also
that, on the whole, the members of the less healthy trades, very poor
persons, habitual drunkards, and others whose prospects of long life
are unfavourable, must (on the average of a large number) be more
likely to remain unmarried than those more favourably situated. Another
fact drawn from the Registrar-General’s return suffices to prove the
influence of poverty on the marriage-rate. I refer to the fact that
marriages are invariably more numerous in seasons of prosperity than
at other times. Improvident marriages are undoubtedly numerous, but
prosperity and adversity _have_ their influence, and that influence not
unimportant, on the marriage returns.
On the other hand, it is perfectly obvious that the life of a married
man is likely to be more favourable to longevity than that of a
bachelor. The mere fact that a man has a wife and family depending
upon him will suffice to render him more careful of his health, less
ready to undertake dangerous employments, and so on; and there are
other reasons which will occur to everyone for considering the life of
a married man better (in the sense of the insurance companies) than
that of a bachelor. In fact, while we are compelled to reject Dr.
Stark’s statement that ‘bachelorhood is more destructive to life than
the most unwholesome trades, or than residence in an unwholesome house
or district, where there has never been the most distant attempt at
sanitary improvement of any kind,’ we may safely accept his opinion
that statistics ‘prove the truth of one of the first natural laws
revealed to man—“It is not good that man should live alone.”’
(From the _Daily News_, October 17, 1868.)
FOOTNOTES:
[14] The reader need hardly be reminded of the complete fulfilment of
this anticipation, during the war between France and Germany.
_THE TOPOGRAPHICAL SURVEY OF INDIA._
At the close of the war with Tippoo Sahib, Major Lambton planned the
triangulation of the country lying between Madras and the Malabar
coast, a district which had been roughly surveyed, during the progress
of the war, by Colonel Mackenzie. The Duke of Wellington gave his
approval to the project, and his brother, the Governor-General of
India, and Lord Clive (son of the great Clive), Governor of Madras,
used their influence to aid Major Lambton in carrying out his design.
The only astronomical instrument made use of by the first survey party
was one of Ramsden’s zenith-sectors, which Lord Macartney had placed in
the hands of Dinwiddie, the astronomer, for sale. A steel chain, which
had been sent with Lord Macartney’s embassy to the Emperor of China and
refused, was the only apparatus available for measuring.
Thus began the great Trigonometrical Survey of India, a work whose
importance it is hardly possible to over-estimate. Conducted
successively by Colonel Lambton, Sir George Everest, Sir Andrew
Waugh, and Lieut.-Col. Walker (the present superintendent), the
trigonometrical survey has been prosecuted with a skill and accuracy
which renders it fairly comparable with the best work of European
surveyors. But to complete in this style the survey of the whole of
India would be the work of several centuries. The trigonometrical
survey of Great Britain and Ireland has been already more than a
century in progress, and is still unfinished. It can, therefore, be
imagined that the survey of India—nearly ten times the size of the
British Isles, and presenting difficulties a hundredfold greater than
those which the surveyor in England has to encounter—is not a work
which can be quickly completed.
But the growing demands of the public service have rendered it
imperatively necessary that India should be rapidly and completely
surveyed. This necessity led to the commencement of the Topographical
Survey of India, a work which has been pushed forward at a surprising
rate during the past few years. My readers may form some idea of the
energy with which the survey is in progress, from the fact that Colonel
Thuillier’s Report for the season 1866-67 announces the charting of an
area half as large as Scotland, and the preparatory triangulation of an
additional area nearly half as large as England.
In a period of thirty years, with but few surveying parties at first,
and a slow increase in their number, an area of 160,000 square miles
has been completed and mapped by the topographical department. The
revenue surveyors have also supplied good maps (on a similar scale)
of 364,000 square miles of country during the twenty years ending in
1866. Combining these results, we have an area of 524,000 square miles,
or upwards of four times that of Great Britain and Ireland. For all
this enormous area the surveyors have the records in a methodical and
systematic form, fit for incorporation in the atlas of India. Nor does
this estimate include the older revenue surveys of the North-western
Provinces which, for want of proper supervision in former years, were
never regularly reduced. The records of these surveys were destroyed
in the Mutiny—chiefly in Hazaumbaugh and the south-western frontier
Agency. The whole of these districts remain to be gone over in a style
very superior to that of the last survey.
The extent of the country which has been charted may lead to the
impression that the survey is little more than a hasty reconnaissance.
This, however, is very far indeed from being the case. The preliminary
triangulation, which is the basis of the topographical survey, is
conducted with extreme care. In the present Report, for instance,
we find that the discrepancies between the common sides of the
triangles-in other words, the discrepancies between the results
obtained by different observers-are in some cases less than one-tenth
of an inch per mile; in others they are from one inch to a foot
per mile; and in the survey of the Cossyah and Garrow Hills, where
observations had to be taken to large objects, such as trees, rocks,
&c., with no defined points for guidance, the results differ by as
much as twenty-six inches per mile. These discrepancies must not only
be regarded as insignificant in themselves, but must appear yet more
trifling when it is remembered that they are not cumulative, inasmuch
as the preliminary triangulation is itself dependent on the great
trigonometrical survey.
Let us understand clearly what are the various forms of survey which
are or have been in progress in India. There are three forms to be
considered:—(1) The Great Trigonometrical Surveys; (2) The Revenue
Surveys; and (3) the Topographical Surveys.
Great trigonometrical operations are extended in a straight course
from one measured base to another. Every precaution which modern skill
and science can suggest is taken in the measurement of each base-line,
and in the various processes by which the survey is extended from one
base-line to the other. The accuracy with which work of this sort
is conducted may be estimated from the following instance. During
the progress of the Ordnance Survey of Great Britain and Ireland,
a base-line nearly eight miles long was measured near Lough Foyle,
in Ireland, and another nearly seven miles long on Salisbury Plain.
Trigonometrical operations were then extended from Lough Foyle to
Salisbury Plain, a distance of about 340 miles; and the Salisbury
base-line was calculated from the observations made over this long
arc. _The difference between the measured and calculated values of
the base-line was less than five inches!_ As we have stated, the
trigonometrical survey of India will bear comparison with the best work
of our surveyors in England.
A revenue survey is prosecuted for the definition of the boundaries of
estates and properties. The operations of such a survey are therefore
carried on conformably to those boundaries.
The topographical survey of a country is defined by Sir A. Scott Waugh
to imply ‘the measurement and delineation of the natural features of a
country, and the works of man thereon, with the object of producing a
complete and sufficiently accurate map. Being free from the trammels
of boundaries of properties, the principal lines of operations must
conform to the features of the country, and objects to be surveyed.’
The only safe basis for the topographical survey of a country is a
system of accurate triangulation. And where the extent of country
to be surveyed is large, there will always be a great risk of the
accumulation of error in the triangulation itself; which must,
therefore, be made to depend on the accurate results obtained by
the great trigonometrical operations. In order to secure this
result, fixed stations are established in the vicinity of the great
trigonometrical series. Where this plan cannot be adopted, a network of
large symmetrical triangles is thrown over the district to be surveyed,
or boundary series of triangles are carried along the outline of the
district or along convenient internal lines. The former of these
methods is applicable to a hilly district, the latter to a flat country.
When the district to be surveyed has been triangulated, the work of
filling-in the topographical details is commenced. Each triangle being
of moderate extent, with sides from three to five miles in length,
and the angular points being determined, as we have seen, with great
exactness, it is evident that no considerable error can occur in
filling-in the details. Hence, methods can be adopted in the final
topographical work which would not be suitable for triangulation. The
triangles can either be ‘measured up,’ or the observer may traverse
from trigonometrical point to point, taking offsets and intersections;
or, lastly, he may make use of the plane table. The two first methods
require little comment; but the principle of plane-tabling enters so
largely into Indian surveying, that this notice would be incomplete
without a brief account of this simple and beautiful method.
The plane-table is a flat board turning on a vertical pivot. It bears
the chart on which the observer is planning the country. Suppose, now,
that two points A and B are determined, and that we require to mark in
the position of a third point C:—It is clear that if we observed with
a theodolite the angles A B C and B A C, we might lay these down on the
chart with a protractor, and so the position of C would be determined,
with an accuracy proportioned to the care with which the observations
were made and the corresponding constructions applied to the chart.
But in ‘plane-tabling’ a more direct plan is adopted. A ruler bearing
sights, resembling those of a rifle, is so applied that the edge
passing through the point A on the chart (the observer being situated
at the real station A) passes through the point B on the chart, the
line of sight passing through the real station B. The table being fixed
in the position thus obtained, the ruler is next directed so that its
edge passes through A, while the line of sight points to C. A line is
now ruled with a pencil through A towards C. In a similar manner, the
table having been removed to the station B, a pencil line is drawn
through the point B on the chart towards C. The two lines thus drawn
determine by their intersection the place of C on the chart.
The above is only one instance of the modes in which a plane-table can
be applied; there are several others. Usually the magnetic compass is
employed to fix the position of the table in accordance with the true
bearing of the cardinal points. Also the bearings of several points
are taken around each station; and thus a variety of tests of the
correctness of the work become applicable. Into such details as these
I need not here enter. It is sufficient that my readers should have
been enabled to recognise the simple principles on which plane-tabling
depends, and the accuracy with which (when suitable precautions are
taken) it can be applied as a method of observation subsidiary to the
ordinary trigonometrical processes.
‘A hilly country,’ says Sir A. Waugh, ‘offers the fairest field for
the practice of plane-table surveys, and the more rugged the surface
the greater will be the relative advantages and facilities this system
possesses over the methods of actual measurement. On the other hand, in
flat lands the plane-table works at a disadvantage, while the traverse
system is facilitated. Consequently, in such tracts, the relative
economy of the two systems does not offer so great a contrast as in
the former. In closely wooded or jungly tracts, all kinds of survey
operations are prosecuted at a disadvantage; but in such localities,
the commanding points must be previously cleared for trigonometrical
operations, which facilitates the use of the table.’
In whatever way the topographical details have been filled in, a
rigorous system of check must be applied to the work. The system
adopted is that of running lines across ground that has been
surveyed. This is done by the head of the party or by the chief
assistant-surveyor. A sufficient number of points are obtained in this
way for comparison with the work of the detail surveyors; and when the
discrepancies exceed certain limits, the work in which they appear
is rejected. Owing to the extremely unhealthy, jungly, and rugged
nature of the ground in which nearly all the Indian surveys have been
progressing, it has not always been found practicable to check by
regularly chained lines. There are, however, other modes of testing
plane-table surveys, and as these entail less labour and expense in
hilly and jungly tracts, and are quite as effective if thoroughly
carried out, they have been adopted generally, while the measured
routes or check-lines have only been pursued under more favourable
conditions. Colonel Thuillier states that ‘the inspection of the work
of every detailed surveyor in the field has been rigorously enforced,
and the work of the field season is not considered satisfactory or
complete unless this duty has been attended to.’
The rules laid down to insure accuracy in the survey are—first, that
the greatest possible number of fixed points should be determined by
regular triangulation; secondly, that the greatest possible number of
plane-table fixings should be made use of within each triangle; and
lastly, that eye-sketching should be reduced to a minimum. If these
rules are well attended to, the surveyor can always rely on the value
of the work performed by his subordinates. But all these conditions
cannot be secured in many parts of the ground allotted to the several
topographical parties owing to the quantity of forest land and the
extremely rugged nature of the country. Hence arises the necessity
for test-lines to verify the details, or for some vigorous system of
check; and this is more especially the case where native assistants are
employed.
So soon as the country has been accurately planned, the configuration
of the ground has to be sketched up. This process is the end and aim of
all the preceding work.
The first point attended to is the arterial system, or water drainage,
constituting the outfall of the country; whence are deduced the lines
of greatest depression of the ground. Next the watersheds or ridges of
hills are traced in, giving the highest level. Lastly, the minor or
subordinate features are drawn in with the utmost precision attainable.
‘The outlines of table-land should be well defined,’ says Sir A. Waugh,
‘and ranges of hills portrayed with fidelity, carefully representing
the watersheds or _divortia aquarum_, the spurs, peaks, depressions or
saddles, isthmuses or connecting-links of separate ranges, and other
ramifications. The depressed points and isthmuses are particularly
valuable, as being either the sites of ordinary passes or points which
new roads should conform to.’
And here we must draw a distinction between survey and reconnaissance.
It is absolutely necessary in making a survey that the outlines of
ground as defined by ridges, water-courses, and feet of hills should
be rigorously fixed by actual observation and careful measurement. In
reconnoitring, more is trusted to the eye.
The scale of the Indian topographical survey is that of one inch per
mile; the scale of half an inch per mile being only resorted to in
very densely wooded or jungly country, containing a few inhabitants
and little cultivated, or where the climate is so dangerous that it is
desirable to accelerate the progress of the survey.
On the scale of one inch per mile the practised draughtsman can survey
about five square miles of average country per day. In intricate
ground, intersected by ravines or covered by hills of irregular
formation, the work proceeds much more slowly; on the other hand, in
open and nearly level country, or where the hills have simple outlines,
the work will cost less and proceed more rapidly. On the scale of one
inch per mile all natural features (such as ravines or watercourses)
more than a quarter of a mile in length can be clearly represented.
Villages, towns, and cities can be shown, with their principal streets
and roads, and the outlines of fortifications. The general figure and
extent of cultivated, waste, and forest lands can be delineated with
more or less precision, according to their extent. Irrigated rice-lands
should be distinctly indicated, since they generally exhibit the
contour of the ground.
The relative heights of hills and depths of valleys should be
determined during the course of a topographical survey. These vertical
elements of a survey can be ascertained by trigonometrical or by
barometrical observations, or by a combination of both methods.
‘The barometer,’ says Sir A. Waugh, ‘is more especially useful
for determining the level of low spots from which the principal
trigonometrical stations are invisible. In using this instrument,
however, in combination with the other operations, the relative
differences of heights are to be considered the quantities sought, so
that all the results may be referable to the original trigonometrical
station. The height above the sea-level of all points coming under any
of the following heads is especially to be determined, for the purpose
of illustrating the physical relief of the country:—
‘1st. The peaks and highest points of ranges.
‘2nd. All obligatory points required for engineering works, such as
roads, drainage, and irrigation, viz.:—the highest points or necks of
valleys; the lowest depressions or passes in ranges; the junctions
of rivers, and _débouchements_ of rivers from ranges; the height of
inundation-level, at moderate intervals of about three miles apart.
‘3rd. Principal towns or places of note.’
Of the various methods employed to indicate the steepness of slope,
that of eye-contouring seems alone to merit special comment. In true
contouring, regular horizontal lines, at fixed vertical intervals,
are traced over a country, and plotted on to the maps. This is an
expensive and tedious process, whereas eye-contouring is easy,
light, and effective. On this system all that is necessary is that
the surveyor should consider what routes persons moving horizontally
would pursue. He draws lines on his chart approximating as closely as
possible to these imaginary lines. It is evident that when lines are
thus drawn for different vertical elevations, the resulting shading
will be dark or light, according as the slope is steep or gentle. This
method of shading affords scope as well for surveying skill as for
draughtsmanship.
(From _Once a Week_, May 1, 1869.)
_A SHIP ATTACKED BY A SWORD-FISH._
I have always been puzzled to imagine how the ‘nine-and-twenty knights
of fame,’ described in the ‘Lay of the Last Minstrel,’ managed to
‘drink the red wine through the helmet barr’d.’ But in nature we meet
with animals which seem almost as inconveniently armed as those chosen
knights, who
... quitted not their armour bright,
Neither by day nor yet by night.
Amongst such animals the sword-fish must be recognised as one of the
most uncomfortably-armed creatures in existence. The shark has to
turn on his back before he can eat, and the attitude scarcely seems
suggestive of a comfortable meal. But the sword-fish can hardly even
by that arrangement get his awkwardly projecting snout out of the way.
Yet doubtless this feature, which seems so inconvenient, is of great
value to Xiphias. In some way as yet unknown it enables him to get his
living. Whether he first kills some one of his neighbours with this
instrument, and then eats him at his leisure, or whether he plunges it
deep into the larger sort of fish, and attaching himself to them in
this way, sucks nutriment from them while they are yet alive, is not
known to naturalists. Certainly, he is fond of attacking whales, but
this may result not so much from gastronomic tastes as from a natural
antipathy—envy, perhaps, at their superior bulk. Unfortunately for
himself, Xiphias, though cold-blooded, seems a somewhat warm-tempered
animal; and, when he is angered, he makes a bull-like rush upon his
foe, without always examining with due care whether he is likely to
take anything by his motion. And when he happens to select for attack
a stalwart ship, and to plunge his horny beak through thirteen or
fourteen inches of planking, with perhaps a stout copper sheathing
outside it, he is apt to find some little difficulty in retreating. The
affair usually ends by his leaving his sword embedded in the side of
the ship. In fact, no instance has ever been recorded of a sword-fish
recovering his weapon (if I may use the expression) after making a
lunge of this sort. Last Wednesday the Court of Common Pleas—rather
a strange place, by-the-bye, for inquiring into the natural history
of fishes—was engaged for several hours in trying to determine under
what circumstances a sword-fish might be able to escape scot-free
after thrusting his snout into the side of a ship, The gallant ship
‘Dreadnought,’ thoroughly repaired, and classed A 1 at Lloyd’s, had
been insured for 3,000_l._ against all the risks of the seas. She
sailed on March 10, 1864, from Colombo, for London. Three days later,
the crew, while fishing, hooked a sword-fish. Xiphias, however,
broke the line, and a few moments after leaped half out of the water,
with the object, it would seem, of taking a look at his persecutor,
the ‘Dreadnought.’ Probably he satisfied himself that the enemy was
some abnormally large cetacean, which it was his natural duty to
attack forthwith. Be this as it may, the attack was made, and at four
o’clock the next morning the captain was awakened with the unwelcome
intelligence that the ship had sprung a leak. She was taken back to
Colombo, and thence to Cochin, where she was hove down. Near the keel
was found a round hole, an inch in diameter, running completely through
the copper sheathing and planking.
As attacks by sword-fish are included among sea risks, the insurance
company was willing to pay the damages claimed by the owners of the
ship, if only it could be proved that the hole had really been made by
a sword-fish. No instance had ever been recorded in which a sword-fish
had been able to withdraw his sword after attacking a ship. A defence
was founded on the possibility that the hole had been made in some
other way. Professor Owen and Mr. Frank Buckland gave their evidence;
but neither of them could state quite positively whether a sword-fish
which had passed its beak through three inches of stout planking could
withdraw without the loss of its sword. Mr. Buckland said that fish
have no power of ‘backing,’ and expressed his belief that he could
hold a sword-fish by the beak; but then he admitted that the fish had
considerable lateral power, and might so ‘wriggle its sword out of a
hole.’ And so the insurance company will have to pay nearly six hundred
pounds because an ill-tempered fish objected to be hooked, and took its
revenge by running full tilt against copper sheathing and oak planking.
(From the _Daily News_, December 11, 1868.)
_THE SAFETY-LAMP._
As recent colliery explosions have attracted a considerable amount
of attention to the principle of the safety-lamp, and questions have
arisen respecting the extent of the immunity which the action of this
lamp secures to the miner, it may be well for me briefly to point out
the true qualities of the lamp.
In the Davy lamp a common oil-light is surrounded by a cylinder of
wire-gauze. When the air around the lamp is pure the flame burns as
usual, and the only effect of the gauze is somewhat to diminish the
amount of light given out by the lamp. But so soon as the air becomes
loaded with the carburetted hydrogen gas generated in the coal-strata,
a change takes place. The flame grows larger and less luminous. The
reason of the change is this:—The flame is no longer fed by the
oxygen of the air, but is surrounded by an atmosphere which is partly
inflammable; and the inflammable part of the gas, so fast as it passes
within the wire cylinder, is ignited and burns within the gauze.
Thus the light now given out by the lamp is no longer that of the
comparatively brilliant oil flame, but is the light resulting from the
combustion of carburetted hydrogen, or ‘fire damp,’ as it is called;
and every student of chemistry is aware that the flame of this gas has
very little illuminating power.
So soon as the miner sees the flame thus enlarged and altered in
appearance he should retire. But it is not true that explosion would
necessarily follow if he did not do so. The danger is great because
the flame within the lamp is in direct contact with the gauze, and if
there is any defect in the wire-work, the heat may make for itself
an opening which—though small—would yet suffice to enable the flame
within the lamp to ignite the gas outside. So long, however, as the
wire-gauze continues perfect, even though it become red-hot, there will
be no explosion. No authority is required to establish this point,
which has been proved again and again by experiment; but I quote
Professor Tyndall’s words on the subject to remove some doubts which
have been entertained on the matter. ‘Although a continuous explosive
atmosphere,’ he says, ‘may extend from the air outside through the
meshes of the gauze to the flame within, ignition is not propagated
across the gauze. The lamp may be filled with an almost lightless
flame; still explosion does not occur. A defect in the gauze, the
destruction of the wire at any point by oxidation hastened by the
flame playing against it, would cause explosion;’ and so on. It need
hardly be said, however, that, imprudent as miners have often been,
no miner would remain where his lamp burned with the enlarged flame
indicative of the presence of fire-damp. The lamp should also be at
once extinguished.
But here we touch on a danger which undoubtedly exists, and—so far as
has yet been seen—cannot be guarded against by any amount of caution.
Supposing the miner sought to extinguish the lamp by blowing it out, an
explosion would almost certainly ensue, since the flame can be forced
mechanically through the meshes, though it will not pass through them
when it is burning in the ordinary way. Now of course no miner who had
been properly instructed in the use of the safety-lamp would commit
such a mistake as this. But it happens, unfortunately, that sometimes
the fire-damp itself forces the flame of the lamp through the meshes.
The gas frequently issues with great force from cavities in the coal
(in which it has been pent up), when the pick of the miner breaks an
opening for it. In these circumstances an explosion is inevitable, if
the issuing stream of gas happen to be directed full upon the lamp.
Fortunately, however, this is a contingency which does not often
arise. It is one of those risks of coal-mining which seem absolutely
unavoidable by any amount of care or caution. It would be well if it
were only such risks as these that the miner had to face.
Another peculiarity sometimes noticed when there is a discharge of
fire-damp is worth mentioning. It happens, occasionally, that the light
will be put out owing to the absolute exclusion of air from the lamp.
This, however, can only happen when the gas issues in so large a volume
that the atmosphere of the pit becomes irrespirable.
With the exception of the one risk which we have pointed out above, the
Davy lamp may be said to be absolutely safe. It is necessary, however,
that caution and intelligence should be exhibited in its use. On this
point Professor Tyndall remarks that unfortunately the requisite
intelligence is not often possessed nor the requisite caution exercised
by the miner, ‘and the consequence is that even with the safety-lamp,
explosions still occur.’ And he suggests that it would be well to
exhibit to the miner in a series of experiments the properties of the
valuable instrument which has been devised for his security. ‘Mere
advice will not enforce caution,’ he says; ‘but let the miner have the
physical image of what he is to expect clearly and vividly before his
mind, and he will find it a restraining and monitory influence long
after the effect of cautioning words has passed away.’
A few words on the history of the invention may be acceptable. Early
in the present century a series of terrible catastrophes in coal
mines had excited the sympathy of enlightened and humane persons
throughout the country. In the year 1813, a society was formed at
Sunderland to prevent accidents in coal mines or at least to diminish
their frequency, and prizes were offered for the discovery of new
methods of lighting and ventilating mines. Dr. William Reid Clanny, of
Bishopwearmouth, presented to this society a lamp which burnt without
explosion in an atmosphere heavily loaded with fire-damp; for which
invention the Society of Arts awarded him a gold medal. The Rev.
Dr. Gray called the attention of Sir Humphry Davy to the subject,
and that eminent chemist visited the coal mines in 1815 with the
object of determining what form of lamp would be best suited to meet
the requirements of the coal miners. He invented two forms of lamp
before discovering the principle on which the present safety-lamps
are constructed. This principle—the property, namely, that flame will
not pass through small apertures—had been, we believe, discovered by
Stephenson, the celebrated engineer, some time before; and a somewhat
angry controversy took place respecting Davy’s claim to the honour
of having invented the safety-lamp. It seems admitted, however, by
universal consent, that Davy’s discovery of the property above referred
to was made independently, and also that he was the first to suggest
the idea of using wire-gauze in place of perforated tin.
In comparing the present frequency of colliery explosions with what
took place before the invention of the safety-lamp, we must take
into consideration the enormous increase in the coal trade since the
introduction of steam machinery. The number of miners now engaged in
our coal mines is far in excess of the number employed at the beginning
of the present century. Thus accidents in the present day are at
once more common on account of the increased rapidity with which the
mines are worked, and when they occur there are more sufferers; so
that the frequency of colliery explosions in the opening years of the
present century and the number of deaths resulting from them, are in
reality much more significant than they seem to be at first sight. But
even independently of this consideration, the record of the colliery
accidents which took place at that time is sufficiently startling.
Seventy-two persons were killed in a colliery at North Biddick at the
commencement of the present century. Two explosions in 1805, at Hepburn
and Oxclose, left no less than forty-three widows and 151 children
unprovided for. In 1808, ninety persons were killed in a coal-pit at
Lumley. On May 24, 1812, ninety-one persons were killed by an explosion
at Felling Colliery, near Gateshead. And many more such accidents might
readily be enumerated.
(From the _Daily News_, December 4, 1868.)
_THE DUST WE HAVE TO BREATHE._
A microscopist, Mr. Dancer, F.R.A.S., has been examining the dust of
our cities. The results are not pleasing. We had always recognised
city dust as a nuisance, and had supposed that it derived the
peculiar grittiness and flintiness of its structure from the constant
macadamizing of city roads. But it now appears that the effects
produced by dust, when, as is usual, it finds its way to our eyes,
our nostrils, and our throats, are as nothing compared with the
mischief it is calculated to produce in a more subtle manner. In every
specimen examined by Mr. Dancer animal life was abundant. But the
amount of ‘molecular activity’—such is the euphuism under which what
is exceedingly disagreeable to contemplate is spoken about—is variable
according to the height at which the dust is collected. And of all
heights which these molecular wretches could select for the display of
their activity, the height of five feet is that which has been found to
be the favourite. Just at the average height of the foot-passenger’s
mouth these moving organisms are always waiting to be devoured and
to make us ill. And this is not all. As if animal abominations were
insufficient, a large proportion of vegetable matter also disports
itself in the light dust of our streets. The observations show that in
thoroughfares where there are many animals engaged in the traffic, the
greater part of the vegetable matter thus floating about ‘consists
of what has passed through the stomachs of animals,’ or has suffered
decomposition in some way or other. This unpleasing matter, like
the ‘molecular activity,’ floats about at a height of five feet, or
thereabouts.
After this, one begins to recognise the manner in which some diseases
propagate themselves. What had been mysterious in the history of
plagues and pestilences seems to receive at least a partial solution.
Take cholera, for example. It has been shown by the clearest and most
positive evidence that this disease is not propagated in any way
save one—that is, by the actual swallowing of the cholera poison. In
Professor Thudichum’s masterly paper on the subject in the ‘Monthly
Microscopical Journal,’ it is stated that doctors have inhaled a full
breathing from a person in the last stage of this terrible malady
without any evil effects. Yet the minutest atom of the cholera poison
received into the stomach will cause an attack of cholera. A small
quantity of this matter drying on the floor of the patient’s room, and
afterwards caused to float about in the form of dust, would suffice to
prostrate a houseful of people. We can understand, then, how matter
might be flung into the streets, and, after drying, its dust be wafted
through a whole district, causing the death of hundreds. One of the
lessons to be learned from these interesting researches of Mr. Dancer
is clearly this, that the watering-cart should be regarded as one
of the most important of our hygienic institutions. Supplemented by
careful scavengering, it might be effective in dispossessing many a
terrible malady which now holds sway from time to time over our towns.
(From the _Daily News_, March 6, 1869.)
_PHOTOGRAPHIC GHOSTS._
On the outskirts of the ever-widening circle lighted up by science
there is always a border-land wherein superstition holds sway. ‘The
arts and sciences may drive away the vulgar hobgoblin of darker days;
but they bring with them new sources of illusion. The ghosts of old
could only gibber; the spirits of our day can read and write, and
play on divers musical instruments, and quote Shakespeare and Milton.
It is not, therefore, altogether surprising to learn that they can
take photographs also. You go to have your photograph taken, we will
suppose, desiring only to see your own features depicted in the
_carte_; and lo! the spirits have been at work, and a photographic
phantom makes its appearance beside you. It is true this phantom is of
a hazy and dubious aspect: the ‘dull mechanic ghost’ is indistinct,
and may be taken for anyone. Still, it is not difficult for the eye
of fancy to trace in it the lineaments of some departed friend, who,
it is to be assumed, has come to be photographed along with you. In
fact, photography, according to the spiritualist, resembles what Byron
called—
The lightning of the mind,
Which out of things familiar, undesigned,
When least we deem of such, calls up to view
The spectres whom no exorcism can bind.
The phenomena of spiritual photography were first observed some
years since, and a set of carte photographs were sent from America
to Dr. Walker, of Edinburgh, in which photographic phantoms were
very obviously, however indistinctly, discernible. More recently an
English photographer noticed a yet stranger circumstance, though he
was too sensible to seek for a supernatural interpretation of it. When
he took a photograph with a particular lens, there could be seen not
only the usual portrait of the sitter, but at some little distance a
faint ‘double,’ exactly resembling the principal image. Superstitious
minds might find this result even more distressing than the phantom
photographic friend. To be visited by the departed through the medium
of a lens, is at least not more unpleasing than to hold converse with
spirits through an ordinary ‘rapping’ medium. But the appearance of a
‘double,’ or ‘fetch,’ has ever been held by the learned in ghostly lore
to signify approaching death.
Fortunately both one and the other appearance can be very easily
accounted for without calling in the aid of the supernatural. At a
recent meeting of the Photographical Society it was shown that an image
may often be so deeply impressed on the glass that the subsequent
cleaning of the plate, even with strong acids, will not completely
remove the picture. When the plate is used for receiving another
picture, the original image makes its reappearance, and as it is too
faint to be recognisable, a highly susceptible imagination may readily
transform it into the image of a departed friend. The ‘double’ is
generated by the well-known property of double refraction, obtained by
a lens under certain circumstances of unequal pressure, or sometimes by
inequalities in the process of annealing. So vanish two ghosts which
might have been more or less troublesome to those who are ready to see
the supernatural in commonplace phenomena. Will the time ever come when
no more such phantoms will remain to be exorcised?
(From the _Daily News_, March 2, 1869.)
_THE OXFORD AND CAMBRIDGE ROWING STYLES._
Whatever opinion we may have of the result of the approaching contest
(1869), there can be no doubt that this year, as in former years, there
is a striking dissimilarity between the rowing styles of the dark blue
and the light blue oarsmen. This dissimilarity makes itself obvious
whether we compare the two boats as seen from the side, or when the
line of sight is directed along the length of either. Perhaps it is in
the latter aspect that an unpractised eye will most readily detect the
difference I am speaking of. Watch the Cambridge boat approaching you
from some distance, or receding, and you will notice in the rise and
fall of the oars, as so seen, the following peculiarities—a long stay
of the oar in the water, a quick rise from and return to the water,
the oars remaining out of the water for the briefest possible interval
of time. In the case of the Oxford boat quite a different appearance
is presented—there is a short stay in the water, a sharp rise from and
return to it, and between these the oars appear to hang over the water
for a perceptible interval. It is, however, when the boats are seen
from the side that the meaning of these peculiarities is detected, and
also that the fundamental distinction between the two styles is made
apparent to the experienced eye. In the Cambridge boat we recognise the
long stroke and ‘lightning feather’ inculcated in the old treatises
on rowing: in the Oxford boat we see these conditions reversed, and
in their place the ‘waiting feather’ and lightning stroke. By the
‘waiting feather’ I do not refer to what is commonly understood by slow
feathering, but to a momentary pause (scarcely to be detected when the
crew is rowing hard) before the simultaneous dash of the oars upon
the first grip of the stroke.[15] And observing more closely—which,
by the way, is no easy matter—as either boat dashes swiftly past, we
detect the distinctive peculiarities of ‘work’ by which the two styles
are severally arrived at. In the Cambridge crew we see the first part
of the stroke done with the shoulders—precisely according to the
old-fashioned models—the arms straight until the body has fallen back
to an almost upright position; then comes the sharp drop back of the
shoulders beyond the perpendicular, the arms simultaneously doing
their work, so that as the swing back is finished, the backs of the
hands just touch the ribs in feathering. All these things are quite in
accordance with what used to be considered the perfection of rowing;
and, indeed, this style of rowing has some important good qualities
and a very handsome appearance. The lightning feather, also, which
follows the long sweeping stroke, is theoretically perfect. Now, in
the case of the Oxford crew, we observe a style which at first sight
seems less excellent. As soon as the oars are dashed down and catch
their first hold of the water, the arms as well as the shoulders of
each oarsman are at work.[16] The result is, that when the back has
reached an upright position, the arms have already reached the chest,
and the stroke is finished. Thus the Oxford stroke takes a perceptibly
shorter time than the Cambridge stroke; it is also, necessarily,
somewhat shorter in the water. One would, therefore, say it must be
less effective. Especially would an unpractised observer form this
opinion, because the Oxford stroke seems to be much shorter in range
than it is in reality. _There_ we have the secret of its efficiency.
It is actually as long as the Cambridge stroke, but is taken in a
perceptibly shorter time. What does this mean but that the oar is taken
more sharply, and, therefore, much more effectively, through the water?
Much more effectively so far as the actual conditions of the contest
are concerned. The modern racing outrigger requires a sharp impulse,
because it will take almost any speed we can apply to it. It will
also retain that speed between the strokes, a consideration of great
importance. The old-fashioned racing-eights required to be continually
under propulsion. The lightning-feather was a necessity in their
case, for between every stroke the boat would lag terribly with a
slow-feathering crew. I do not say, of course, that the speed of a
light outrigged craft does not diminish between the strokes. Anyone who
has watched a closely contested bumping-race, and noticed the way in
which the sharply cut bow of the pursuing boat draws up to the rudder
of the other as by a succession of impulses, although either boat seen
alone would seem to sweep on with almost uniform speed, will know that
the motion of the lightest boat is not strictly uniform. But there is
an immense difference between the almost imperceptible loss of way of a
modern eight and the dead ‘lag’ in the old-fashioned craft. And hence
we get the following important consideration. Whereas with the old
boats it was useless for a crew to attempt to give a very quick motion
to their boat by a sharp, sudden ‘lift,’ this plan is calculated to be,
of all others, the most effective with the modern racing-eight.
It may seem, at first sight, that, after all, the result of the
Cambridge style should be as effective as that of the other. If arms
and shoulders do their work in both crews with equal energy—which we
may assume to be the case—and if the number of strokes per minute is
equal, the actual propulsive energy ought to be equal likewise. A
little consideration will show that this is a fallacy. If two men pull
at a weight together they will move it farther with a given expenditure
of energy than if first one and then the other apply his strength to
the work. And what is more to the purpose, they will be able to move
it faster. So shoulders and arms working simultaneously will give a
greater propulsive power than when working separately, even though in
the latter case each works with its fullest energy. And not only so,
but by the simultaneous use of arms and shoulders, that sharpness of
motion can alone be given which is essential to the propulsion of a
modern racing-boat.
I have said that the two crews are severally rowing in the style
which has lately been peculiar to their respective Universities. But
the Cambridge crew is rowing in that form of the Cambridge style which
brings it nearest to the requirements of modern racing. The faults of
the style are subdued, so to speak, and its best qualities brought
out effectively. In one or two of the long series of defeats lately
sustained by Cambridge the reverse has been the case. At present, too,
there is a certain roughness about the Oxford crew which encourages the
hopes of the light blue supporters. But it must be admitted that this
roughness is rather apparent than real, great as it seems, and it will
doubtless disappear before the day of encounter. I venture to predict
that the ‘time’ of the approaching race, taken in conjunction with the
state of the tide, will show the present crews to be at least equal to
the average.[17]
(From the _Daily News_, April 1869.)
FOOTNOTES:
[15] The grip is never properly caught without the pause; but anything
beyond a momentary pause is a bad fault in style.
[16] I write this with full knowledge that many Oxford men deny the
fact. I have rowed behind Cambridge, Oxford, and London strokes, and
have several times taken the place (number 2 thwart) of a London
waterman in a four (‘stroked’ by John Mackinney) training for the
Thames Regatta. So that I have had ample opportunities for comparing
different rowing styles; and I am satisfied that the main defect of
the real Cambridge style was (and perhaps is) an _exaggeration_ of the
sound rule that a boat should be propelled rather by the body than
by the arms. The very swing in a Cambridge boat shows that this must
be so. On the other hand, the Thames watermen do too much arm-work;
and hence seem to double a little over their oars. I once rowed with
some Cambridge friends from London nearly to Oxford and back, taking a
Thames waterman as ‘help.’ We set him, at first, for our strokesman,
but we soon had to make him row _bow_, for we could none of us stand
his gripping, arm-working style.
_BETTING ON HORSE RACES: OR, THE STATE OF THE ODDS._
There appears every day in the newspapers an account of the betting
on the principal forthcoming races. The betting on such races as the
Two Thousand Guineas, the Derby, and the Oaks, often begins more than
a year before the races are run; and during the interval, the odds
laid against the different horses engaged in them vary repeatedly, in
accordance with the reported progress of the animals in their training,
or with what is learned respecting the intentions of their owners. Many
who do not bet themselves, find an interest in watching the varying
fortunes of the horses which are held by the initiated to be leading
favourites, or to fall into the second rank, or merely to have an
outside chance of success. It is amusing to notice, too, how frequently
the final state of the odds is falsified by the event; how some ‘rank
outsider’ will run into the first place, while the leading favourites
are not even ‘placed.’
It is in reality a simple matter to understand the betting on races (or
contests of any kind), yet it is astonishing how seldom those who do
not actually bet upon races have any inkling of the meaning of those
mysterious columns which indicate the opinion of the betting world
respecting the probable results of approaching contests, equine or
otherwise.
Let us take a few simple cases of ‘odds,’ to begin with; and, having
mastered the elements of our subject, proceed to see how cases of
greater complexity are to be dealt with.
Suppose the newspapers inform us that the betting is 2 to 1 against
a certain horse for such and such a race, what inference are we to
deduce? To learn this let us conceive a case in which the _true_ odds
against a certain event are as 2 to 1. Suppose there are three balls
in a bag, one being white, the others black. Then, if we draw a ball
at random, it is clear that we are twice as likely to draw a black as
to draw a white ball. This is technically expressed by saying that
the odds are 2 to 1 _against_ drawing a white ball; or 2 to 1 _on_
(that is, in favour of) drawing a black ball. This being understood,
it follows that, when the odds are said to be 2 to 1 against a certain
horse, we are to infer that, in the opinion of those who have studied
the performance of the horse, and compared it with that of the other
horses engaged in the race, his chance of winning is equivalent to the
chance of drawing one particular ball out of a bag of three balls.
Observe how this result is obtained: the odds are 2 to 1, and the
chance of the horse is as that of drawing one ball out of a bag of
three—three being the sum of the two numbers 2 and 1. This is the
method followed in all such cases. Thus, if the odds against a horse
are 7 to 1, we infer that the _cognoscenti_ consider his chance equal
to that of drawing one particular ball out of a bag of _eight_.
A similar treatment applies when the odds are not given as so many to
_one_. Thus, if the odds against a horse are as 5 to 2, we infer that
the horse’s chance is equal to that of drawing a white ball out of a
bag containing five black and two white balls—or seven in all.
We must notice also that the number of balls may be increased to any
extent, provided the proportion between the total number and the
number of a specified colour remains unchanged. Thus, if the odds are
5 to 1 against a horse, his chance is assumed to be equivalent to that
of drawing _one_ white ball out of a bag containing six balls, only one
of which is white; _or_ to that of drawing a white ball out of a bag
containing sixty balls, of which ten are white-and so on. This is a
very important principle, as we shall now see.
Suppose there are two horses (amongst others) engaged in a race,
and that the odds are 2 to 1 against one, and 4 to 1 against the
other-what are the odds that one of the two horses will win the race?
This case will doubtless remind my readers of an amusing sketch by
Leech, called—if I remember rightly—‘Signs of the Commission.’ Three
or four undergraduates are at a ‘wine,’ discussing matters equine. One
propounds to his neighbour the following question: I say, Charley,
if the odds are 2 to 1 against _Rataplan_, and 4 to 1 against _Quick
March_, what’s the betting about the pair?’—‘Don’t know, I’m sure,’
replies Charley; ‘but I’ll give you 6 to 1 against them.’ The absurdity
of the reply is, of course, very obvious; we see at once that the odds
cannot be heavier against a pair of horses than against either singly.
Still, there are many who would not find it easy to give a correct
reply to the question. What has been said above, however, will enable
us at once to determine the just odds in this or any similar case.
Thus-the odds against one horse being 2 to 1, his chance of winning is
equal to that of drawing one white ball out of a bag of _three_, one
only of which is white. In like manner, the chance of the second horse
is equal to that of drawing one white ball out of a bag of _five_,
one only of which is white. Now we have to find a number which is a
multiple of both the numbers three and five. Fifteen is such a number.
The chance of the first horse, modified according to the principle
explained above, is equal to that of drawing a white ball out of a bag
of fifteen of which _five_ are white. In like manner, the chance of
the second is equal to that of drawing a white ball out of a bag of
fifteen of which _three_ are white. Therefore the chance that _one of
the two_ will win is equal to that of drawing a white ball out of a bag
of fifteen balls of which _eight_ (_five_ added to _three_) are white.
There remain _seven_ black balls, and therefore the odds are 8 to 7
_on_ the pair.
To impress the method of treating such cases on the mind of the reader,
let us take the betting about three horses—say 3 to 1, 7 to 2, and 9
to 1 _against_ the three horses respectively. Then their respective
chances are equal to the chance of drawing (1) one white ball out of
_four_, one only of which is white; (2) a white ball out of _nine_,
of which two only are white; and (3) one white ball out of _ten_, one
only of which is white. The least number which contains four, nine, and
ten is 180; and the above chances, modified according to the principle
explained above, become equal to the chance of drawing a white ball out
of a bag containing 180 balls, when 45, 40, and 18 (respectively) are
white. Therefore, the chance that one of the three will win is equal
to that of drawing a white ball out of a bag containing 180 balls, of
which 103 (the sum of 45, 40, and 18) are white. Therefore, the odds
are 103 to 77 _on_ the three.
One does not hear in practice of such odds as 103 to 77. But
betting-men (whether or not they apply just principles of computation
to such questions, is unknown to me) manage to run very near the truth.
For instance, in such a case as the above, the odds on the three would
probably be given as 4 to 3—that is, instead of 103 to 77 (or 412 to
308), the published odds would be equivalent to 412 to 309.
And here a certain nicety in betting has to be mentioned. In running
the eye down the list of odds, one will often meet such expressions as
10 to 1 against such a horse _offered_, or 10 to 1 _wanted_. Now, the
odds of 10 to 1 _taken_ may be understood to imply that the horse’s
chance is equivalent to that of drawing a certain ball out of a bag
of eleven. But if the odds are offered and not taken, we cannot infer
this. The offering of the odds implies that the horse’s chance is _not
better_ than that above mentioned, but the fact that they are not taken
implies that the horse’s chance is _not so good_. If no higher odds are
offered against the horse, we may infer that his chance is _very little
worse_ than that mentioned above. Similarly, if the odds of 10 to 1 are
_asked for_, we infer that the horse’s chance is _not worse_ than that
of drawing one ball out of eleven; if the odds are not obtained, we
infer that his chance is _better_; and if no lower odds are asked for,
we infer that his chance is _very little better_.
Thus, there might be _three_ horses (A, B, and C) against whom the
nominal odds were 10 to 1, and yet these horses might not be equally
good favourites, because the odds might not be taken, or might be asked
for in vain. We might accordingly find three such horses arranged thus:—
Odds.
A 10 to 1 (wanted).
B 10 to 1 (taken).
C 10 to 1 (offered).
Or these different stages might mark the upward or downward progress
of the same horse in the betting. In fact, there are yet more delicate
gradations, marked by such expressions respecting certain odds,
as—_offered freely_, _offered_, _offered and taken_ (meaning that some
offers only have been accepted), _taken_, _taken and wanted_, _wanted_,
and so on.
As an illustration of some of the principles I have been considering,
let us take from the day’s paper,[18] the state of the odds respecting
the ‘Two Thousand Guineas.’ It is presented in the following form:—
TWO THOUSAND GUINEAS.
7 to 2 against _Rosicrucian_ (off.).
6 to 1 against _Pace_ (off.; 7 to 1 w.).
10 to 1 against _Green Sleeve_ (off.).
100 to 7 against _Blue Gown_ (off.).
180 to 80 against Sir J. Hawley’s lot (t.).
This table is interpreted thus: bettors are willing to lay the same
odds against _Rosicrucian_ as would be the true mathematical odds
against drawing a white ball out of a bag containing two white and
seven black balls; but no one is willing to back the horse at this
rate; on the other hand, higher odds are not offered against him. Hence
it is presumable that his chance is somewhat less than that above
indicated. Again, bettors are willing to lay the same odds against
_Pace_ as might fairly be laid against drawing one white ball out of
a bag of seven, one only of which is white; but backers of the horse
consider that they ought to get the same odds as might be fairly laid
against drawing the white ball when an additional black ball had been
put into the bag. As respects _Green Sleeve_ and _Blue Gown_, bettors
are willing to lay the odds which there would be, respectively, against
drawing a white ball out of a bag containing—(1) eleven balls, one only
of which is white, and (2) one hundred and seven balls, seven only of
which are white. Now, the three horses, _Rosicrucian_, _Green Sleeve_,
and _Blue Gown_, all belong to Sir Joseph Hawley, so that the odds
about the three are referred to in the last statement of the list just
given. And since none of the offers against the three horses have been
taken, we may expect the odds actually taken about ‘Sir Joseph Hawley’s
lot’ to be more favourable than those obtained by summing up the three
former in the manner we have already examined. It will be found that
the resulting odds (offered) against Sir J. Hawley’s lot—estimated in
this way—should be, as nearly as possible, 132 to 80. We find, however,
that the odds _taken_ are 180 to 80. Hence, we learn that the offers
against some or all of the three horses are considerably short of what
backers require; or else that some person has been induced to offer far
heavier odds against Sir J. Hawley’s lot than are justified by the fair
odds against his horses, severally.
I have heard it asked why a horse is said to be a favourite, though the
odds may be against him. This is very easily explained. Let us take as
an illustration the case of a race in which four horses are engaged to
run. If all these horses had an equal chance of winning, it is very
clear that the case would correspond to that of a bag containing four
balls of different colours; since, in this case, we should have an
equal chance of drawing a ball of any assigned colour. Now, the odds
against drawing a particular ball would clearly be 3 to 1. This, then,
should be the betting against each of the three horses. If any one of
the horses has less odds offered against him, he is _a favourite_.
There may be more than one of the four horses thus distinguished; and,
in that case, the horse against which the least odds are offered is
_the first favourite_. Let us suppose there are two favourites, and
that the odds against the leading favourite are 3 to 2, those against
the other 2 to 1, and those against the best non-favourite 4 to 1; and
let us compare the chances of the four horses. I have not named any
odds against the fourth, because, if the odds against all the horses
but one are given, the just odds against that one are determinable,
as we shall see immediately. The chance of the leading favourite
corresponds to the chance of drawing a ball out of a bag in which are
three black and two white balls, _five_ in all; that of the next to
the chance of drawing a ball out of a bag in which are two black and
one white ball, _three_ in all; that of the third, to the chance of
drawing a ball out of a bag in which are four black balls and one white
one, _five_ in all. We take, then, the least number containing both
five and three—that is, _fifteen_; and then the number of white balls,
corresponding to the chances of the three horses, are respectively six,
five, and three, or fourteen in all; leaving only _one_ to represent
the chance of the fourth horse (against which the odds are therefore
14 to 1). Hence the chances of the four horses are respectively as the
numbers _six_, _five_, _three_ and _one_.
I have spoken above of the published odds. The statements made in the
daily papers commonly refer to wagers actually made, and therefore
the uninitiated might suppose that everyone who tried would be able
to obtain the same odds. This is not the case. The wagers which are
laid between practised betting-men afford very little indication of
the prices which would be forced (so to speak) upon an inexperienced
bettor. Book-makers—that is, men who make a series of bets upon
several or all of the horses engaged in a race—naturally seek to give
less favourable terms than the known chances of the different horses
engaged would suffice to warrant. As they cannot offer such terms to
the initiated, they offer them-and in general success—fully—to the
inexperienced.
It is often said that a man may so lay his wagers about a race as to
make sure of gaining money whichever horse wins the race. This is not
strictly the case. It is of course possible to make sure of winning if
the bettor can only get persons to lay or take the _odds he requires to
the amount he requires_. But this is precisely the problem which would
remain insoluble if all bettors were equally experienced.
Suppose, for instance, that there are three horses engaged in a race
with equal chances of success. It is readily shown that the odds are
2 to 1 against each. But if a bettor can get a person to take even
betting against the first horse (A), a second person to do the like
about the second horse (B), and a third to do the like about the third
horse (C), and if all these bets are made to the same amount—say
1000_l._—then, inasmuch as only one horse can win, the bettor loses
1000_l._ on that horse (say A), and gains the same sum on each of the
two horses B and C. Thus, on the whole, he gains 1000_l._, the sum laid
out against each horse.
If the layer of the odds had laid the true odds to the same amount
on each horse, he would neither have gained nor lost. Suppose, for
instance, that he laid 1000_l._ to 500_l._ against each horse, and A
won; then he would have to pay 1000_l._ to the backer of A, and to
receive 500_l._ from each of the backers of B and C. In like manner,
a person who had backed each horse to the same extent would neither
lose nor gain by the event. Nor would a backer or layer who had wagered
_different_ sums _necessarily_ gain or lose by the race; he would gain
or lose _according to the event_. This will at once be seen, on trial.
Let us next take the case of horses with unequal prospects of
success—for instance, take the case of the four horses considered
above, against which the odds were respectively 3 to 2, 2 to 1, 4 to
1, and 14 to 1. Here, suppose the same sum laid against each, and for
convenience let this sum be 84_l._ (because 84 contains the numbers 3,
2, 4, and 14). The layer of the odds wagers 84_l._ to 56_l._ against
the leading favourite, 84_l._ to 42_l._ against the second horse,
84_l._ to 21_l._ against the third, and 84_l._ to 6_l._ against the
fourth. Whichever horse wins, the layer has to pay 84_l._; but if
the favourite wins, he receives only 42_l._ on one horse, 21_l._ on
another, and 6_l._ on the third—that is 69_l._ in all, so that he loses
15_l._; if the second horse wins, he has to receive 56_l._, 21_l._,
and 6_l._—or 83_l._ in all, so that he loses 1_l._; if the third horse
wins, he receives 56_l._, 42_l._, and 6_l._—or 104_l._ in all, and thus
gains 20_l._; and lastly, if the fourth horse wins, he has to receive
56_l._, 42_l._, and 2l_l._—or 119_l._ in all, so that he gains 35_l._
He clearly risks much less than he has a chance (however small) of
gaining. It is also clear that in all such cases the worst event for
the layer of the odds is, that the favourite should win. Accordingly,
as professional book-makers are nearly always layers of odds, one often
finds the success of a favourite spoken of in the papers as a ‘great
blow for the book-makers,’ while the success of a rank outsider will be
described as ‘a misfortune to backers.’
But there is another circumstance which tends to make the success of a
favourite a blow to layers of the odds and _vice versâ_. In the case we
have supposed, the money actually pending about the four horses (that
is, the sum of the amount laid _for_ and _against_ them) was 140_l._
as respects the favourite, 126_l._ as respects the second, 105_l._
as respects the third, and 90_l._ as respects the fourth. But as a
matter of fact the amounts pending about the favourites bear always a
much greater proportion than the above to the amounts pending about
outsiders. It is easy to see the effect of this. Suppose, for instance,
that instead of the sums 84_l._ to 56_l._, 84_l._ to 42_l._, 84_l._
to 21_l._, and 84_l._ to 6_l._, a book-maker had laid 8400_l._ to
5600_l._, 840_l._ to 420_l._, 84_l._ to 21_l._, and 14_l._ to 1_l._,
respectively—then it will easily be seen that he would lose 7958_l._
by the success of the favourite; whereas he would gain 4782_l._ by
the success of the second horse, 5937_l._ by that of the third, and
6027_l._ by that of the fourth. I have taken this as an extreme case;
as a general rule, there is not so great a disparity as has been here
assumed between the sums pending on favourites and outsiders.
Finally, it may be asked whether, in the case of horses having unequal
chances, it is possible that wagers can be so proportioned (just odds
being given and taken), that, as in the former case, a person backing
or laying against all the four shall neither gain nor lose. It is so.
All that is necessary is, that the sum actually pending about each
horse shall be the same. Thus, in the preceding case, if the wagers
9_l._ to 6_l._, 10_l._ to 5_l._, 12_l._ to 3_l._, and 14_l._ to 1_l._,
are either laid or taken by the same person, he will neither gain nor
lose by the event, whatever it may be. And therefore, if unfair odds
are laid or taken about all the horses, in such a manner that the
amounts pending on the several horses are equal (or nearly so), the
unfair bettor must win by the result. Say, for instance, that instead
of the above odds, he lays 8_l._ to 6_l._, 9_l._ to 5_l._, 11_l._ to
3_l._ and 13_l._ to 1_l._, against the four horses respectively; it
will be found that he _must_ win 1_l._ Or if he _takes_ the odds 18_l._
to 11_l._, 20_l._ to 9_l._, 24_l._ to 5_l._, and 28_l._ to 1_l._ (the
just odds being 18_l._ to 12_l._, 20_l._ to 10_l._, 24_l._ to 6_l._,
and 28_l._ to 2_l._ respectively), he will win 1_l._ by the race. So
that, by giving or taking such odds to a sufficiently great amount, a
bettor would be certain of pocketing a large sum, whatever the event of
a given race might be.
In every instance, a man who bets on a race _must risk his money_,
unless he can succeed in taking unfair advantages over those with whom
he bets. My readers will conceive how small must be the chance that an
unpractised bettor will gain anything but dearly-bought experience by
speculating on horse-races. I would recommend those who are tempted
to hold another opinion to follow the plan suggested by Thackeray in
a similar case—to take _a good look_ at professional and practised
betting-men, and to decide ‘which of those men they are most likely to
get the better of’ in turf transactions.
(From _Chambers’s Journal_, July 1869.)
FOOTNOTES:
[17] The race (that of 1869) was one of the best ever rowed, and the
time of the winners (Oxford) better than in any former race.
[18] This article was written early in March 1868.
_SQUARING THE CIRCLE._
There must be a singular charm about insoluble problems, since there
are never wanting persons who are willing to attack them. I doubt not
that at this moment there are persons who are devoting their energies
to Squaring the Circle, in the full belief that important advantages
would accrue to science—and possibly a considerable pecuniary profit
to themselves—if they could succeed in solving it. Quite recently,
applications have been made to the Paris Academy of Sciences, to
ascertain what was the amount which that body was authorised to pay
over to anyone who should square the circle. So seriously, indeed,
was the secretary annoyed by applications of this sort, that it was
found necessary to announce in the daily journals that not only was
the Academy not authorised to pay any sum at all, but that it had
determined never to give the least attention to those who fancied they
had mastered the famous problem.
It is a singular circumstance that people have even attacked the
problem without knowing exactly what its nature is. One ingenious
workman, to whom the difficulty had been propounded, actually set to
work to invent an arrangement for measuring the circumference of the
circle; and was perfectly satisfied that he had thus solved a problem
which had mastered all the mathematicians of ancient and modern times.
That we may not fall into a similar error, let us clearly understand
what it is that is required for the solution of the problem of
‘squaring the circle.’
To begin with, we must note that the term ‘squaring the circle’ is
rather a misnomer; because the true problem to be solved is the
determination of the length of a circle’s circumference when the
diameter is known. Of course, the solution of this problem, or, as it
is termed, the _rectification_ of the circle, involves the solution of
the other, or the _quadrature_, of the circle. But it is well to keep
the simpler issue before us.
Many have supposed that there exists some exact relation between the
circumference and the diameter of the circle, and that the problem to
be solved is the determination of this relation. Suppose, for example,
that the approximate relation discovered by Archimedes (who found, that
if a circle’s diameter is represented by _seven_, the circumference may
be almost exactly represented by _twenty-two_) were strictly correct,
and that Archimedes had proved it to be so; then, according to this
view, he would have solved the great problem; and it is to determine
a relation of some such sort that many persons have set themselves.
Now, undoubtedly, if any relation of this sort could be established,
the problem would be solved; but as a matter of fact no such relation
exists, and the solution of the problem does not require that there
should be any relation of the sort. For example, we do not look on the
determination of the diagonal of a square (whose side is known) as an
insoluble, or as otherwise than a very simple problem. Yet in this case
no exact relation exists. We cannot possibly express both the side
and the diagonal of a square in whole numbers, no matter what unit of
measurement we adopt: or, to put the matter in another way, we cannot
possibly divide both the side and the diagonal into equal parts (which
shall be the same along each), no matter how small we take the parts.
If we divide the side into 1,000 parts, there will be 1,414 such parts,
_and a piece over_ in the diagonal; if we divide the side into 10,000
parts, there will be 14,142, and still a little piece over, in the
diagonal; and so on for ever. Similarly, the mere fact that no exact
relation exists between the diameter and the circumference of a circle
is no bar whatever to the solution of the great problem.
Before leaving this part of the subject, however, I may mention a
relation which is very easily remembered, and is very nearly exact—much
more so, at any rate, than that of Archimedes. Write down the
numbers 113,355, that is, the first three odd numbers each repeated
twice over. Then separate the six numbers into two sets of three,
thus,—113) 355, and proceed with the division thus indicated. The
result, 3·1415929..., expresses the circumference of a circle whose
diameter is 1, correctly to the sixth decimal place, the true relation
being 3·14159265.
Again, many people imagine that mathematicians are still in a state of
uncertainty as to the relation which exists between the circumference
and the diameter of the circle. If this were so, scientific societies
might well hold out a reward to anyone who could enlighten them; for
the determination of this relation (with satisfactory exactitude) may
be held to lie at the foundation of the whole of our modern system of
mathematics. I need hardly say that no doubt whatever rests on the
matter. A hundred different methods are known to mathematicians by
which the circumference may be calculated from the diameter with any
required degree of exactness. Here is a simple one, for example:—Take
any number of the fractions formed by putting _one_ as a numerator over
the successive odd numbers. Add together the alternate ones beginning
with the first, which, of course, is unity. Add together the remainder.
Subtract the second sum from the first. The remainder will express
the circumference (the diameter being taken as unity) to any required
degree of exactness. We have merely to take enough fractions. The
process would, of course, be a very laborious one, if great exactness
were required, and as a matter of fact mathematicians have made use of
much more convenient methods for determining the required relation:
but the method is strictly exact.
The largest circle we have much to do with in scientific questions is
the earth’s equator. As a matter of curiosity, we may inquire what the
circumference of the earth’s orbit is; but as we are far from being
sure of the exact length of the radius of that orbit (that is, of the
earth’s distance from the sun), it is clear that we do not need a very
exact relation between the circumference and the diameter in dealing
with that enormous circle. Confining ourselves, therefore, to the
circle of the earth’s equator, let us see what exactness we seem to
require. We will suppose for a moment that it is possible to measure
round the earth’s equator without losing count of a single yard, and
that we want to gather from our estimate what the diameter of this
great circle may be. This seems, indeed, the only use to which, in
this case, we can put our knowledge of the relation we are dealing
with. We have then a circle some twenty-five thousand miles round,
and each mile contains one thousand seven hundred and sixty yards: or
in all there are some forty-four million yards in the circumference,
and therefore (roughly) some fourteen million yards in the diameter
of this great circle. Hence, if our relation is correct within a
fourteen-millionth part of the diameter, or a forty-four millionth part
of the circumference, we are safe from any error exceeding a yard. All
we want, then, is that the number expressing the circumference (the
diameter being unity) should be true to the eighth decimal place, as
quoted above (p. 291, l. 5).
But as I have said, mathematicians have not been content with a
computation of this sort. They have calculated the number not to the
_eighth_, but to the _six hundred and twentieth_ decimal place. Now,
if we remember that each new decimal makes the result ten times more
exact, we shall begin to see what a waste of time there has been in
this tremendous calculation. We all remember the story of the horse
which had twenty-four nails in its shoes, and was valued at the sum
obtained by adding together a farthing for the first nail, a halfpenny
for the next, a penny for the next, and so on, doubling twenty-four
times. The result was counted by thousands of pounds. The old miser who
paid at a similar rate for a grave eighteen feet deep (doubling for
each foot), killed himself when he heard the total. But now consider
the effect of multiplying by ten, six hundred and twenty times. A
fraction, with that enormous number for denominator, and unity for
numerator, expresses the minuteness of the error which would result
if the ‘long value’ of the circumference were made use of. Let an
illustration show the force of this:—
It has been estimated that light, which could eight times circle the
earth in a second, takes 50,000 years in reaching us from the faintest
stars seen in Lord Rosse’s giant reflector. Suppose we knew the exact
length of the tremendous line which extends from the earth to such a
star, and wanted, for some inconceivable purpose, to know the length
of the circumference of a circle, of which that line was the radius.
The value deduced from the above-mentioned calculation of the relation
between the circumference and the diameter would differ from the truth
by a length which would be imperceptible under the most powerful
microscope ever yet constructed. Nay, the radius we have conceived,
enormous as it is, might be increased a million-fold, or a million
times a million-fold, with the same result. And the area of the circle
formed with this increased radius would be determinable with so much
accuracy, that the error, if presented in the form of a minute square,
would be utterly imperceptible under a microscope a million times more
powerful than the best ever yet constructed by man.
Not only has the length of the circumference been calculated once in
this unnecessarily exact manner, but a second calculator has gone over
the work independently. The two results are of course identical figure
for figure.
It will be asked then, what _is_ the problem about which so great a
work has been made? The problem is, in fact, utterly insignificant; its
only interest lies in the fact that it is insoluble—a property which it
shares along with many other problems, as the trisection of an angle,
the duplication of a cube, and so on.
The problem is simply this: _Having given the diameter of a circle,
to determine, by a geometrical construction, in which only straight
lines and circles shall be made use of, the side of a square, equal in
area to the circle_. As I have said, the problem is solved, if, by a
construction of the kind described, we can determine the length of the
circumference; because then the rectangle under half this length and
the radius is equal in area to the circle, and it is a simple problem
to describe a square equal to a given rectangle.
To illustrate the kind of construction required, I give an approximate
solution which is remarkably simple, and, so far as I am aware, not
generally known. Describe a square about the given circle, touching it
at the ends of two diameters, AOB, COB, at right angles to each other,
and join CA; let COAE be one of the quarters of the circumscribing
square, and from E draw EG, cutting off from AO a fourth part AG
of its length, and from AC the portion AH. Then three sides of the
circumscribing square together with AH are very nearly equal to the
circumference of the circle. The difference is so small, that in a
circle two feet in diameter, it would be less than the two-hundredth
part of an inch. If this construction were exact, the great problem
would have been solved.
One point, however, must be noted; the circle is of all curved lines
the easiest to draw by mechanical means. But there are others which can
be so drawn. And if such curves as these be admitted as available, the
problem of the quadrature of the circle can be readily solved. There
is a curve, for instance, invented by Dinostratus, which can readily
be described mechanically, and has been called the quadratrix of
Dinostratus, because it has the property of thus solving the problem we
are dealing with.
As such curves can be described with quite as much accuracy as the
circle—for, be it remembered, an absolutely perfect circle has never
yet been drawn—we see that it is only the limitations which geometers
have themselves invented that give this problem its difficulty. Its
solution has, as I have said, no value; and no mathematician would ever
think of wasting a moment over the problem—for this reason, simply,
that it has long since been demonstrated to be insoluble by simple
geometrical methods. So that, when a man says he has squared the circle
(and many will say so, if one will only give them a hearing), he shows
that either he wholly misunderstands the nature of the problem, or that
his ignorance of mathematics has led him to mistake a faulty for a true
solution.
(From _Chambers’s Journal_, January 16, 1869.)
_A NEW THEORY OF ACHILLES’ SHIELD._
A distinguished classical authority has remarked that the description
of Achilles’ shield occupies an anomalous position in Homer’s ‘Iliad.’
On the one hand, it is easy to show that the poem—for the description
may be looked on as a complete poem—is out of place in the ‘Iliad;’ on
the other, it is no less easy to show that Homer has carefully led up
to the description of the shield by a series of introductory events.
I propose to examine, briefly, the evidence on each of these points,
and then to exhibit a theory respecting the shield which may appear
_bizarre_ enough on a first view, but which seems to me to be supported
by satisfactory evidence.
An argument commonly urged against the genuineness of the ‘Shield
of Achilles’ is founded on the length and laboured character of the
description. Even Grote, whose theory is that Homer’s original poem
was not an _Iliad_, but an _Achilleis_, has admitted the force of this
argument. He finds clear evidence that from Book II. to Book XX. Homer
has been husbanding his resources for the more effective description
of the final conflict. He therefore concedes the possibility that
the ‘Shield of Achilles’ may be an interpolation—perhaps the work of
another hand.
It appears to me, however, that the mere length of the description
is no argument against the genuineness of the passage. Events have,
indeed, been hastening to a crisis up to the end of Book XVII., and the
action is checked in a marked manner by the ‘Oplopœia’ in Book XVIII.
Yet it is quite in Homer’s manner to introduce, between two series of
important events, an interval of comparative inaction, or at least of
events wholly different in character from those of either series. We
have a marked instance of this in Books IX. and X. Here the appeal to
Achilles and the night-adventure of Diomed and Ulysses are interposed
between the first victory of the Trojans and the great struggle in
which Patroclus is slain, and Agamemnon, Ulysses, Diomed, Machaon,
and Eurypylus wounded.[19] In fact, one cannot doubt that in such an
arrangement Homer exhibits admirable taste and judgment. The contrast
between action and inaction, or between the confused tumult of a heady
conflict and the subtle advance of the two Greek heroes, is conceived
in the true poetic spirit. The dignity and importance of the action,
and the interest of the interposed events, are alike enhanced. Indeed,
there is scarcely a noted author whose works do not afford instances of
corresponding contrasts. How skilfully, for example, has Shakespeare
interposed the ‘bald, disjointed chat’ of the sleepy porter between
the conscience-wrought horror of Duncan’s murderers and the ‘horror,
horror, horror’ which ‘tongue nor heart could not conceive nor name’
of his faithful followers. Nor will the reader need to be reminded of
the frequent and effective use of the contrast between the humorous and
the pathetic by others.
The laboured character of the description of the shield is an
argument—though not, perhaps, a very striking one—for the independent
origin of the poem.
But the arguments on which I am disposed to lay most stress lie nearer
the surface.
Scarcely anyone, I think, can have read the description of the shield
without a feeling of wonder that Homer should describe the shield of a
mortal hero as adorned with so many and such important objects. We find
the sun and moon, the constellations, the waves of ocean, and a variety
of other objects, better suited to adorn the temple of a great deity
than the shield of a warrior, however noble and heroic. The objects
depicted even on the Ægis of Zeus are much less important. There is
certainly no trace in the ‘Iliad’ of a wish on Homer’s part to raise
the dignity of mortal heroes at the expense of Zeus, yet the Ægis is
thus succinctly described:—
Fring’d round with ever-fighting snakes, though it was drawn to life,
The miseries and deaths of fight; in it frown’d bloody Strife,
In it shone sacred Fortitude, in it fell Pursuit flew,
In it the monster Gorgon’s head, in which held out to view
Were all the dire ostents of Jove.—_Chapman’s_ Translation.
Five lines here, as in the original, suffice for the description of
Jove’s Ægis, while one hundred and thirty lines are employed in the
description of the celestial and terrestrial objects depicted on the
shield of Achilles.
Another circumstance attracts notice in the description of Achilles’
armour—the disproportionate importance attached to the shield.
Undoubtedly, the shield was that portion of a hero’s armour which
admitted of the freest application of artistic skill. Yet this
consideration is not sufficient to account for the fact, that while so
many lines are given to the shield, the helmet, corselet, and greaves
are disposed of in four.
But the argument on which I am inclined to lay most stress is the
occurrence _elsewhere_ of a description which is undoubtedly only
another version of the ‘Shield of Achilles.’ The ‘Shield of Hercules’
occurs in a poem ascribed to Hesiod. But whatever opinion may be
formed respecting the authorship of the description, there can be
no doubt that it is not Hesiod’s work. It exhibits no trace of his
dry, didactic, somewhat heavy style. Elton ascribes the ‘Shield of
Hercules’ to an imitator of Homer, and in support of this view points
out those respects in which the poem resembles, and those in which it
is inferior to, the ‘Shield of Achilles.’ The two descriptions are,
however, absolutely identical in many places; and this would certainly
not have happened if one had been an honest imitation of the other. And
those parts of the ‘Shield of Hercules,’ which have no counterparts
in the ‘Shield of Achilles,’ are too well conceived and expressed
to be ascribed to a very inferior poet—a poet so inferior as to be
reduced to the necessity of simply reproducing Homer’s words in other
parts of the poem. Those parts which admit of comparison—where, for
instance, the same objects are described, but in different terms—are
certainly inferior in the ‘Shield of Hercules.’ The description is
injured by the addition of unnecessary or inharmonious details. Elton
speaks, accordingly, of these portions as if they were expansions of
the corresponding parts of the ‘Shield of Achilles.’ This appears to
me a mistake. It seems far more likely that both descriptions are by
the same poet. It is not necessary for the support of my theory that
this poet should be Homer, but I think both descriptions show undoubted
traces of his handiwork. Indeed, all known imitations of Homer are so
easily recognisable as the work of inferior poets, that I should have
thought no doubt could exist on this point, but for the attention which
the German theory respecting the ‘Iliad’ has received. Assigning both
poems to Homer, the ‘Shield of Hercules’ may be regarded, not as an
expansion (in parts) of the ‘Shield of Achilles,’ but as an earlier
work of Homer’s, improved and pruned by his maturer judgment, when
he desired to fit it into the plan of the ‘Iliad.’ Or rather, each
poem may be looked on as an abridgment (the ‘Shield of Hercules’ the
earlier) of an independent work on a subject presently to be mentioned.
It is next to be shown that in the events preceding the ‘Oplopœia,’
there is a preparation for the introduction of a separate poem.
In the first place, every reader of Homer is familiar with the
fact that the poet constantly makes use, when occasion serves, of
expressions, sentences, often even of complete passages, which have
been already applied in a corresponding, or occasionally even in a
wholly different relation. The same epithets are repeatedly applied
to the same deity or hero. A long message is delivered in the very
words which have been already used by the sender of the message. In
one well-known instance (in Book II.), not only is a message delivered
thus, but the person who has received it repeats it to others in
precisely the same terms. In the combat between Hector and Ajax (Book
VI.), the flight of Ajax’s spear and the movement by which Hector
avoids the missile, are described in six lines, differing only as to
proper names from those which had been already used in describing the
encounter between Paris and Menelaus (Book III.).
This peculiarity would be a decided blemish in a written poem.
Tennyson, indeed, occasionally copies Homer’s manner—for instance, in
‘Enid,’ he twice repeats the line—
As careful robins eye the delver’s toil;—
but with a good taste which prevents the repetition from becoming
offensive. The fact is, that the peculiarity marks Homer as the
_singer_, not the _writer_, of poetry. I would not be understood as
accepting the theory, according to which the ‘Iliad’ is a mere string
of ballads. I imagine that no one who justly appreciates that noble
poem would be willing to countenance such a theory. But that the whole
poem was sung by Homer at those prolonged festivals which formed a
characteristic peculiarity of Achaian manners seems shown, not only by
what we learn respecting the later ‘rhapsodists,’ but by the internal
evidence of the poem itself.[20]
Homer, reciting a long and elaborate poem of his own composition,
occasionally varying the order of events, or adding new episodes,
extemporized as the song proceeded, would exhibit the peculiarity
invariably observed in the ‘improvisatore,’ of using, more than once,
expressions, sentences, or passages which happened to be conveniently
applicable. The art of extemporizing depends on the capacity for
composing fresh matter while the tongue is engaged in the recital of
matter already composed. Anyone who has watched a clever improvisatore
cannot fail to have noticed that, though gesture is aptly wedded to
words, the thoughts are elsewhere. In the case, therefore, of an
improvisatore, or even of a rhapsodist reciting from memory, the
occasional recurrence of a well-worn form of words serves as a relief
to the strained invention or memory.
We have reason then for supposing that if Homer had, in his earlier
days, composed a poem which was applicable, with slight alterations,
to the story of the ‘Iliad,’ he would endeavour, by a suitable
arrangement of the plan of his narrative, to introduce the lines whose
recital had long since become familiar to him.
Evidence of design in the introduction of the ‘Shield of Achilles’
certainly does not seem wanting.
It is by no means necessary to the plot of the ‘Iliad’ that Achilles
should lose the celestial armour given to Peleus as a dowry with
Thetis. On the contrary, Homer has gone out of his way to render
the labours of Vulcan necessary. Patroclus has to be so ingeniously
disposed of, that while the armour he had worn is seized by Hector, his
body is rescued, as are also the horses and chariot of Achilles.
We have the additional improbability that the armour of the great
Achilles should fit the inferior warriors Patroclus and Hector. Indeed,
that the armour should fit Hector, or rather that Hector should fit the
armour, the aid of Zeus and Ares has to be called in—
To this Jove’s sable brows did bow; and he made fit his limbs
To those great arms, to fill which up the war-god enter’d him
Austere and terrible, his joints and every part extends
With strength and fortitude.—_Chapman’s_ Translation.
It is clear that the narrative would not have been impaired in any way,
while its probability and consistency would have been increased, if
Patroclus had fought in his own armour. The death of Patroclus would in
any case have been a cause sufficient to arouse the wrath of Achilles
against Hector—though certainly the hero’s grief for his armour is
nearly as poignant as his sorrow for his friend.
It appears probable, then, that the description of Achilles’ Shield is
an interpolation—the poet’s own work, however, and brought in by him
in the only way he found available. The description clearly refers to
the same object which is described (here, also, only in part) in the
‘Shield of Hercules.’ The original description, doubtless, included all
that is found in both ‘shields,’ and probably much more.
What, then, was the object to which the original description applied?
An object, I should think, far more important than a warrior’s shield.
I imagine that anyone who should read the description without being
aware of its accepted interpretation, would consider that the poet was
dealing with an important series of religious sculptures, possibly
that he was describing the dome of a temple adorned with celestial and
terrestrial symbols.
In Egypt there are temples of a vast antiquity, having a dome, on which
a zodiac—or, more correctly, a celestial hemisphere—is sculptured with
constellation-figures. And we now learn, from ancient Babylonian and
Assyrian sculptures, that these Egyptian zodiacs are in all probability
merely copies (more or less perfect) of yet more ancient Chaldæan
zodiacs. One of these Babylonian sculptures is figured in Rawlinson’s
‘Ancient Monarchies.’ It seems probable that in a country where
Sabæanism, or star-worship, was the prevailing form of religion, yet
more imposing proportions would be given to such zodiacs than in Egypt.
My theory, then, respecting the shield of Achilles is this—
I conceive that Homer, in his eastern travels, visited imposing
temples devoted to astronomical observation and star-worship; and
that nearly every line in both ‘shields’ is borrowed from a poem
in which he described a temple of this sort, its domed zodiac, and
those illustrations of the labours of different seasons and of
military or judicial procedures which the astrological proclivities
of star-worshippers led them to associate with the different
constellations.
I think there are arguments of some force to be urged in support of
this theory, fanciful as it may seem at a first view.
In the first place, it is necessary that the constellations recognised
in Homer’s time (not necessarily, or probably, _by_ Homer) should be
distinguished from later inventions.
Aratus, writing long after Homer’s date, mentions forty-five
constellations. These were probably derived, without exception, from
the globe of Eudoxus. Remembering the tendency which astronomers have
shown, in all ages, to add to the list of constellations, we may
assume that in Homer’s time the number was smaller. Probably there
were some fifteen northern and ten southern constellations, besides
the twelve zodiacal signs. The smaller constellations mentioned by
Aratus doubtless formed parts of larger figures. Anyone who studies the
heavens will recognise the fact that the larger constellations have
been robbed of their just proportions to form the smaller asterisms.
Corona Borealis was the right arm of Bootes, Ursa Minor was a wing of
Draco (now wingless, and no longer a dragon), and so on.
Secondly, it is necessary that the actual appearance of the heavens,
with reference to the position of the pole in Homer’s time should be
indicated. For my present purpose, it is not necessary that we should
know the exact date at which the most ancient of the zodiac-temples
were constructed (or to which they were made to correspond). There are
good reasons, though this is not the proper place for dwelling upon
them, for supposing that the great epoch of reference amongst ancient
astronomers preceded the Christian era by about 2200 years. Be this
as it may, any epoch between the date named and the probable date at
which Homer flourished—say nine or ten centuries before the Christian
era—will serve equally well for my present purpose. Now if the effects
of equinoctial precession be traced back to such a date, we are led to
notice two singular and not uninteresting circumstances. First, the
pole of the heavens fell in the central part of the great constellation
Draco; and, secondly, the equator fell along the length of the great
sea-serpent Hydra, in one part of its course, and elsewhere to the
north of all the ancient aquatic constellations,[21] save that
one-half of the northernmost fish (of the zodiac pair) lay north of
the equator. Thus, if a celestial sphere were constructed with the
equator in a horizontal position, the Dragon would be at the summit,
Hydra would be extended horizontally along the equator—but with his
head and neck reared above that circle—and Argo, Cetus, Capricornus,
Piscis Australis, and Pisces—save one-half of the northernmost—would
lie _below_ the equator. It may also be mentioned that all the
bird-constellations were then, as now, clustered together not far from
the equator—Cygnus (the farthest from the equator) being ten degrees or
so nearer to that circle than at present.
Now let us turn to the two ‘shields,’ and see whether there is anything
to connect them with zodiac-temples, or to remind us of the relations
exhibited above. To commence with the ‘Shield of Achilles,’ the opening
lines inform us that the shield showed—
The starry lights that heav’n’s high convex crown’d,
The Pleiads, Hyads, with the northern team,
And great Orion’s more refulgent beam.
And here, in Achilles’ shield, the list of constellations closes;
but it is remarkable that in the ‘Shield of Hercules,’ while the
above lines are wanting, we find lines which clearly point to other
constellations. Remembering what has just been stated about Draco,
it seems at the least a singular coincidence that we should find the
centre or boss of the shield occupied by a dragon:—
The scaly horror of a dragon, coil’d
Full in the central field, unspeakable,
With eyes oblique retorted, that aslant
Shot gleaming flame.[22]—_Elton’s_ Translation.
We seem, also, to find a reference to the above-named relations of the
aquatic constellations, and specially to the constellation Pisces:—
In the midst,
Full many dolphins chased the fry, and show’d
As though they swam the waters, to and fro
Darting tumultuous: two[23] of silver scale
Panting above the wave.
For we learn from both ‘shields’ that the waves of ocean were figured
in a position corresponding with the above-mentioned position of the
celestial equator, beneath which—that is, _in the ocean_, on our
assumption—the aquatic constellations were figured. The description
of the ocean in the ‘Shield of Hercules’ contains also some lines, in
which we seem to see a reference to the bird-constellations close above
the equator:—
Rounding the utmost verge the ocean flow’d
As in full swell of waters, and the shield
All variegated with whole circle bound.
Swans of high-hovering wing there clamour’d shrill,
Who also skimm’d the breasted surge with plume
Innumerous; near them fishes midst the waves
Frolick’d in wanton bounds.
In the ‘Shield of Achilles’ no mention is made of Perseus, but in the
‘Shield of Hercules’ this well-known constellation seems described in
the lines—
There was the knight of fair-hair’d Danae born,
Perseus; nor yet the buckler with his feet
Touch’d nor yet distant hover’d, strange to see,
For nowhere on the surface of the shield
He rested; so the crippled artist-god
Illustrious fram’d him with his hands in gold.
Bound to his feet were sandals wing’d; a sword
Of brass, with hilt of sable ebony,
Hung round him from the shoulders by a thong.
. . . . . . . . The visage grim
Of monstrous Gorgon all his back o’erspread;
. . . . . . . . the dreadful helm
Of Pluto clasp’d the temples of the prince.
I think that one may recognise a reference to the twins Castor and
Pollux (the wrestler and boxer of mythology) in the words—
But in another part
Were men who wrestled, or in gymnic fight
Wielded the cestus.
Orion is not mentioned by name in the ‘Shield of Hercules,’ as in the
other; but Orion, Lepus, and the two dogs seem referred to:—
Elsewhere men of chase
Were taking the fleet hares; two keen-toothed dogs
Hounded beside; these ardent in pursuit,
Those with like ardour doubling in their flight.
In each ‘shield’ we find a reference to the operations of the
year—hunting and pasturing, sowing, ploughing, and harvesting. It is
hardly necessary to point out the connection between these operations
and astronomical relations. That this connection was fully recognised
in ancient times is shown in the ‘Works and Days’ of Hesiod. We find
also in Egyptian zodiacs clear evidence that these operations, as well
as astronomical symbols or constellations, were pictured in sculptured
domes.
The judicial, military, and other proceedings described in the ‘Shield
of Achilles’ were also supposed by the ancients to have been influenced
by the courses of the stars.
If there were no evidence that ancient celestial spheres presented
the constellations above referred to, I might be disposed to attach
less weight to the coincidences here presented; but the ‘Phenomena’ of
Aratus affords sufficient testimony on this point. In the first place,
that work is of great antiquity, since Aratus flourished two centuries
and a half before the Christian era; but it is well known that Aratus
did not describe the results of his own observations. The positions
of the constellations, as recorded by him, accord neither with the
date at which he wrote nor with the latitude in which he lived. It is
generally assumed—chiefly on the authority of Hipparchus—that Aratus
borrowed his knowledge of astronomy from the sphere of Eudoxus; but we
must go much farther back even than the date of Eudoxus, before we can
find any correspondence between the appearance of the heavens and the
description given by Aratus. Thus we may very fairly assume that the
_origin_ of the constellations (as distinguished from their association
with certain circles of the celestial sphere) may be placed at a date
preceding, perhaps by many generations, that at which Homer flourished.
Indeed, there have not been wanting those who find in the ancient
constellations the record of the early history of man. According to
their views, Orion is Nimrod—the ‘Giant,’ as the Arabic name of the
constellation implies—the mighty hunter, as the dogs and hare beside
him signify. The Centaur bearing a victim towards the altar is Noah;
Argo, the stern of a ship, is the ark, as of old it might be seen on
Mount Ararat. Corvus is the crow sent forth by Noah, and the bird
is placed on Hydra’s back to show that there was no land on which
it could set its foot. The figure now called Hercules, but of old
Engonasin, or the kneeler, and described by Aratus as ‘a man doomed to
labour,’ is Adam. His left foot treads on the dragon’s head, in token
of the saying, ‘It shall bruise thy head; ‘and Serpentarius, or the
serpent-bearer, is the promised seed.
Of course, if we accept these views, we have no difficulty in
understanding that a poet so ancient as Homer should refer to the
constellations which still appear upon celestial spheres. And, in any
case, the mere question of antiquity presents, as we have already
shown, little difficulty.
But there is one difficulty, a notice of which must close this paper,
already carried far beyond the limits I had proposed to myself:—It may
be thought remarkable that heroes of Greek mythology, as Perseus and
Orion, should be placed by Homer, or even by Aratus, in spheres which
are undoubtedly of eastern origin.
Now it may be remarked, first, of Homer, that many acute critics
consider the whole story of the ‘Iliad’ to be, in reality, merely an
adaptation of an eastern narrative to Greek scenes and names. It is
pointed out, that, whereas the Catalogue in Book II. reckons upwards
of 100,000 men, only 10,000 fought at Marathon; and, whereas there are
counted no less than 1,200 ships in the Catalogue, there were but 271
at Artemisium, and at Salamis but 378. However this may be, we have the
distinct evidence of Herodotus that the Greek mythology was derived
originally from foreign sources. He says, ‘All the names of the gods
in Greece were brought from Egypt,’ an opinion in which Diodorus and
other eminent authorities concur. But it is the opinion of acute modern
critics that we must go beyond Egyptian—to Assyrian, or Indian, perhaps
even to Hebrew sources—for the origin of Greek mythology. Layard has
ascribed to Niebuhr the following significant remarks: ‘There is a want
in Grecian art which neither I, nor any man now alive, can supply.
There is not enough in Egypt to account for the peculiar art and the
peculiar mythology which we find in Greece. That the Egyptians did not
originate it I am convinced, though neither I, nor any man now alive,
can say who were the originators. But the time will come when, on the
borders of the Tigris and Euphrates, those who come after me will live
to see the origin of Grecian art and Grecian mythology.’
(From _The Student_, June 1868.)
LONDON: PRINTED BY
SPOTTISWOODE AND CO., NEW-STREET SQUARE
AND PARLIAMENT STREET
FOOTNOTES:
[19] Another well-known instance, where ‘Patroclus, sent in hot haste
for news by a man of the most fiery impatience, is button-held by
Nestor, and though he has no time to sit down, yet is obliged to endure
a speech of 152 lines,’ is accounted for by Gladstone in a different
manner.
[20] Besides Homer’s reference, both in the ‘Iliad’ and ‘Odyssey,’ to
poetic recitations at festivals, there is the well-known invocation in
Book II. To what purpose would the mere writer of poetry pray for an
increase of his physical powers? Nothing could be more proper, says
Gladstone, if Homer were about to recite; nothing less proper if he
were engaged on a written poem.
[21] We may exclude Delphinus as probably later than Homer’s time,
though mentioned by Aratus.
[22] Compare the description of the constellation Draco by Aratus:—
Swol’n is his neck—eyes charg’d with sparkling fire
His crested head illume. As if in ire
To Helice he turns his foaming jaw
And darts his tongue, barb’d with a blazing star.
—_Lamb’s_ Translation.
[23] It is scarcely necessary to remark that, no importance is to
be attached to the numerical relations in this and other passages.
In the original work describing a zodiac-dome, the exact number of
constellations representing fishes, dogs, or the like, would of
course be mentioned; but any changes necessary to Homer’s purpose in
describing a shield would unhesitatingly have been introduced by him
subsequently. It is singular, however, that we should have here, and
in the passage quoted farther on as referring to Orion and the Dogs,
the number _two_ specially mentioned. The latter instance is the more
remarkable inasmuch as the mention of men and hares would lead one to
expect that more than two dogs would be introduced. I would suggest as
a sufficient reason for this peculiarity that the verbal alterations
necessary to pluralise some of the objects in the dome would be more
easily effected than those necessary to undualise others.
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Transcriber's Notes
Obvious typographical errors have been silently corrected. Variations
in hyphenation and all other spelling and punctuation remain unchanged.
Italics are represented thus _italic_.
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