EARTHQUAKES AND VOLCANOES.
Interior of the Earth—Illustration from Norway—Solids and
Liquids—Rigidity of the Interior of the Earth—Earthquakes,
how caused—Their Testimony as to the Rigidity of the
Earth—Delicate Instrument for Measuring Earthquake Tremors—The
Seismometer—Professor Milne’s Work in the Isle of Wight—Different
Earthquake Groups—Precursors and Echoes—Vibrations transmitted
through the Earth’s Centre—Earthquakes in England—Other Evidence of
the Earth’s Rigidity—Krakatoa, August 27th, 1883—The Sounds from
Krakatoa—The Diverging Waves—The Krakatoa Dust—The Hurricane
Overhead—Strange Signs in the Heavens—The Blood-red Skies.
IN this chapter we shall learn what we can as to the physical condition
of the interior of our earth so far as it may be reasonably inferred
from the facts of observation. We have already explained in the last
chapter that a very high temperature must be found at the depth of even
a small fraction of the earth’s radius, and we have pointed out that the
excessively high pressure characteristic of the earth’s interior must be
borne in mind in any consideration as to the condition of the matter
there found.
Let us take, for instance, that primary question in terrestrial physics,
as to whether the interior of the earth is liquid or solid. If we were
to judge merely from the temperatures reasonably believed to exist at a
depth of some twenty miles, and if we might overlook the question of
pressure, we should certainly say that the earth’s interior must be in a
fluid state. It seems at least certain that the temperatures to be found
at depths of two score miles, and still more at greater depths, must be
so high that the most refractory solids, whether metals or minerals,
would at once yield if we could subject them to such temperatures in our
laboratories. At such temperatures every metal would become fluid, even
if it were not transformed into a cloud of vapour. But none of our
laboratory experiments can tell us whether, under the pressure of
thousands of tons on the square inch, the application of any heat
whatever would be adequate to transform solids into liquids. It may
indeed be reasonably doubted whether the terms solids and liquids are
applicable, in the sense in which we understand them, to the materials
forming the interior of the earth.
It was my good fortune some years ago to enjoy a most interesting trip
to Norway, in company with a distinguished geologist. Under his guidance
I there saw evidence which demonstrates conclusively that, when
subjected to great pressure, solids, as we should call them, behave in a
manner which, if not that of actual liquids, resembles at all events in
some of its characteristics the behaviour of liquids. These rocks in
some places are conglomerates, of which the leading constituents are
water-worn pebbles of granite. These pebbles are of various sizes, from
marbles to paving-stones. In some parts of the country these granite
pebbles remain in the form which they acquired on the beach on which
they were rolled by the primæval ocean; in other parts of the same
interesting region the form of the pebbles has been greatly changed from
what it was originally. For in the course of geological periods, and
after the pebbles had become consolidated into the conglomerate, the
rock so formed had been in some cases submitted to enormous pressure.
This may have been lateral pressure, such as is found to have occurred
in many other places, where it has produced the well-known geological
phenomenon of strata crumpled into folds. In the present case, however,
it seemed more probable that it was the actual weight of the
superincumbent rocks, which once lay over these beds of conglomerate,
which produced the surprising transformation. It seems to be not at all
improbable that at one time these beds of conglomerate must have been
covered with strata of which the thickness is so great that it may
actually be estimated by miles. There has, however, been immense
denudation of the superficial rocks in this part, at all events, of
Norway, so that in the course of ages these strata, overlying the
conglomerate for ages, have been so far worn away, and indeed removed,
by the action of ice and the action of water that the conglomerate is
now exposed to view. It offers for our examination striking indications
of the enormous pressure to which it was subjected during the
incalculable ages of geological time.
The effect of this long continuance of great pressure upon the pebbles
of the conglomerate in certain parts of the country has been most
astonishing. The granite in the pebbles still retains its characteristic
crystalline structure; it has obviously not undergone anything that
could be described as fusion; yet under the influence of the two factors
of that pressure, namely, its intensity and its long continuance, the
granite pebbles have yielded. In some cases they are slightly elongated,
in others they are much elongated, while in yet others they are even
rolled out flat. At different places along the valley the various phases
of the transformation can be studied. We can find places where the
pebbles seem little altered, and then we can trace each stage until the
solid granite pebbles have, by the application of excessive pressure,
been compressed into thin sheets whose character it would not have been
easy to divine if it had not been possible to trace out their history.
These sheets lie close and parallel, so that the material thus produced
acquires some of the characteristics of slate. It splits easily along
the flattened sheets, and this rolled-out conglomerate is indeed
actually used as a substitute for slate, and in some places there are
houses roofed with the conglomerate which has been treated in this
extraordinary fashion.
This fact will illustrate a principle, already well known in the arts,
that many, if not all, solids may be made to flow like liquids if only
adequate pressure be applied. The making of lead tubes is a well-known
practical illustration of the same principle, for these tubes are simply
formed by forcing solid lead by the hydraulic press through a mould
which imparts the desired form.
If then a solid can be made to behave like a liquid, even with such
pressures as are within our control, how are we to suppose that the
solids would behave with such pressures as those to which they are
subjected in the interior of the earth? The fact is that the terms solid
and liquid, at least as we understand them, appear to have no physical
meaning with regard to bodies subjected to these stupendous pressures,
and this must be carefully borne in mind when we are discussing the
nature of the interior of the earth.
It must, however, be admitted that the interior of the earth in its
actual physical state seems to possess at least one of the most
important characteristics of a solid, for it seems to be intensely
rigid. We mean by this, that the material of the earth, or rather each
particle of that material, is very little inclined to move from its
position with reference to the adjacent particles by the application of
force. Possibly a liquid, such as water, might not behave very
differently in this respect from a solid such as cast iron, if each of
them were exposed to a pressure of scores of thousands of tons per
square inch, as are the materials which form the great bulk of the
earth. But, without speculating on these points, we are able to
demonstrate that the earth, as a whole, does exhibit extreme rigidity.
This is one of the most remarkable discoveries which has ever been made
with regard to the physics of our earth. The discovery that the earth is
so rigid is mainly due to Lord Kelvin.
We shall now mention the line of evidence which appears to prove, in the
simplest and most direct manner, the excessive rigidity of our earth. It
is derived from the study of earthquake phenomena, and we must endeavour
to set it forth with the completeness its importance deserves.
As to the immediate cause of earthquakes, there is no doubt considerable
difference of opinion. But I think it will not be doubted that an
earthquake is one of the consequences, though perhaps a remote one, of
the gradual loss of internal heat from the earth. As this terrestrial
heat is gradually declining, it follows from the law that we have
already so often had occasion to use that the bulk of the earth must be
shrinking. No doubt the diminution in the earth’s diameter, due to the
loss of heat must be excessively small, even in a long period of time.
The cause, however, is continually in operation, and accordingly the
crust of the earth has, from time to time, to be accommodated to the
fact that the whole globe is lessening. The circumference of our earth
at the Equator must be gradually declining; a certain length in that
circumference is lost each year. We may admit that loss to be a quantity
far too small to be measured by any observations as yet obtainable, but,
nevertheless, it is productive of phenomena so important that it cannot
be overlooked.
It follows from these considerations that the rocks which form the
earth’s crust over the surface of the continents and the islands, or
beneath the beds of ocean, must have a lessening acreage year by year.
These rocks must therefore submit to compression, either continuously or
from time to time, and the necessary yielding of the rocks will in
general take place in those regions where the materials of the earth’s
crust happen to have comparatively small powers of resistance. The acts
of compression will often, and perhaps generally, not proceed with
uniformity, but rather with small successive shifts, and even though the
displacements of the rocks in these shifts be actually very small, yet
the pressures to which the rocks are subjected are so vast that a very
small shift may correspond to a very great terrestrial disturbance.
Suppose, for instance, that there is a slight shift in the rocks on each
side of a crack, or fault, at a depth of ten miles. It must be
remembered that the pressure ten miles down would be about thirty-five
tons on the square inch. Even a slight displacement of one extensive
surface over another, the sides being pressed together with a force of
thirty-five tons on the square inch, would be an operation necessarily
accompanied by violence greatly exceeding that which we might expect
from so small a displacement if the forces concerned had been only of
more ordinary magnitude. On account of this great multiplication of the
intensity of the phenomenon, merely a small rearrangement of the rocks
in the crust of the earth, in pursuance of the necessary work of
accommodating its volume to the perpetual shrinkage, might produce an
excessively violent shock extending far and wide. The effect of such a
shock would be propagated in the form of waves through the globe, just
as a violent blow given at one end of a bar of iron by a hammer is
propagated through the bar in the form of waves. When the effect of this
internal adjustment reaches the earth’s surface, it will sometimes be
great enough to be perceptible in the shaking it gives that surface. The
shaking may be so violent that buildings may not be able to withstand
it. Such is the phenomenon of an earthquake.
Earthquakes have been made to yield testimony of the most striking
character with regard to the rigidity of the earth. The researches we
are now to describe are mainly due to Professor Milne, who, having
enjoyed the advantage of studying earthquakes in their natural home in
Japan, where are to be found some of the most earthquake-shaken regions
of this earth, has now transferred his observations of these phenomena
to the more peaceful regions of the Isle of Wight. But though the Isle
of Wight is perhaps one of the last places in the world to which anyone
who desired to experience violent earthquake shocks would be likely to
go, yet by the help of a beautiful apparatus Professor Milne is actually
able to witness important earthquakes that are happening all over the
world. He has a demonstration of these earthquakes in the indications of
an extremely sensitive instrument which he has erected in his home at
Shide.
When our earth is shaken by one of those occasional adjustments of the
crust which I have described, the wave that spreads like a pulsation
from the centre of agitation extends all over our globe and, indeed I
may say, is transmitted right through it. At the surface lying
immediately over the centre of disturbance there will be a violent
shock. In the surrounding country, and often over great distances, the
earthquake may also be powerful enough to produce destructive effects.
The convulsion may also be manifested over a far larger area of country
in a way which makes the shock to be felt, though the damage wrought may
not be appreciable. But beyond a limited distance from the centre of the
agitation the earthquake will produce no destructive effects upon
buildings, and will not even cause vibrations that would be appreciable
to ordinary observation.
This earth of ours may transmit from an earthquake pulses of a very
distinct and definite character, which are too weak to be perceived by
our unaided senses; but, just as the microscope will render objects
visible which are too minute to be perceived without this aid to the
ordinary vision, so these faint earth-pulses may be rendered perceptible
by the delicate indications of an instrument which perceives and records
tremors that would pass unnoticed by our ordinary observations. The
ingenious instrument for studying earthquakes is called a seismometer.
It marks on a revolving drum of paper the particulars of those
infinitesimal tremors by which the earth is almost daily agitated in one
place or another.
Let us suppose, for example, that an earthquake occurs in Japan, in
which much agitated country it is, I believe, estimated that no fewer
than one thousand earthquakes of varying degrees of intensity occur
annually in one district or another. Let us suppose that this earthquake
behaves as serious earthquakes usually do; that it knocks down buildings
and monuments, causes landslips, raises great waves in the sea and hurls
them as inundations on the land. We may also suppose that it issues
tragically in the loss of many lives and that there is a destruction of
much property, and that its energies in the acutely violent form extend
over, let us say, an area of a hundred square miles. Beyond that area of
greatest destruction such an earthquake would be felt over a great
extent of country as a shaking more or less vehement, and characteristic
rumbling sounds would be heard. But the intensity declines with the
distance, and we may feel confident that not even the faintest
indications of the earthquake would be perceptible by the unaided senses
at a thousand miles from its origin. A thousand miles is, however, less
than a fifth of the distance between Tokio and Shide, in the Isle of
Wight, measured in a great circle round the earth’s surface. The acutest
sense could not perceive the slightest indication of the convulsion in
Japan at even half the distance between these two places. But the earth
transmits so faithfully the undulations committed to its care that
though the intensity may have declined so as to be no longer perceptible
to the unaided sense, it is still possible that they may be shown
distinctly on the seismometer in Professor Milne’s laboratory, even
after a journey of five thousand miles. This instrument not only
announces that an earthquake has been in progress some little time
previously, but the recording pencil reproduces with marvellous fidelity
some actual details of the vibration. The movements of the line up and
down on the revolving drum of paper show how the convulsions succeed
each other, and their varying intensity. Thus Professor Milne is enabled
to set down some features of the earthquake long before the post brings
an account of the convulsion from the unhappy locality.
Professor Milne’s account of work in studying earthquakes has the charm
of a romance, even while it faithfully sets out the facts of Nature. I
have supposed the earthquake to take place in Japan; but we must observe
that the seismometer at Shide will also take account of considerable
earthquakes in whatever part of the world the disturbance may arise.
There are, for example, localities in the West Indies in which
earthquakes are by no means infrequent, though they may not be phenomena
of almost daily occurrence, as they are in Japan. Every considerable
earthquake, no matter where its centre may lie, produces in our whole
globe a vibration or a tingle which is sufficient to be manifested by
the delicate indications of the seismometer at Shide. Thus this
instrument, which in the morning may record an earthquake from Japan,
will in the afternoon of the same day delineate with equal fidelity an
earthquake from the opposite hemisphere in the neighbourhood of the
Caribbean Sea.
In each locality in which earthquakes are chronic it would seem as if
there must be some particularly weak spot in the earth some miles below
the surface. A shrinkage of the earth, in the course of the incessant
adjustment between the interior and the exterior, will take place by
occasional little jumps at this particular centre. The fact that there
is this weak spot at which small adjustments are possible may provide,
as it were, a safety-valve for other places in the same part of the
world. Instead of a general shrinking, the materials would be
sufficiently elastic and flexible to allow the shrinking for a very
large area to be done at this particular locality. In this way we may
explain the fact that immense tracts on the earth are practically free
from earthquakes of a serious character, while in the less fortunate
regions the earthquakes are more or less perennial.
The characteristics of an earthquake record, a _seismogram_, if we give
it the correct designation, depend on the distance of the origin from
the locality where the record is made. The length of the journey, as
might be expected, tells on the character of the inscription which the
earthquake waves make on the drum. If, for instance, the first
intimation of a large earthquake received at Shide precedes the second
by about thirty-five minutes, it may be concluded that the earthquake
has come from Japan.
In like manner the shocks, with their origin in the West Indies, will
proceed from their particular earthquake centre, and consequently all
the earthquakes from this source will possess a characteristic
resemblance. The Japan group of earthquakes will have, so to speak, a
family resemblance; and the Trinidad group of earthquakes, though quite
different from the Japan group, will also possess a family resemblance.
These features are faithfully transmitted by undulations through the
earth and round the earth; thus in due course they reach the Isle of
Wight, and they are reproduced by the pencil of the seismometer. The
different earthquakes of a family may differ in size, in intensity, and
undulation, but they will have the features appropriate to the
particular group from which they come. From long experience Professor
Milne has become so familiar with the lineaments of these earthquake
families, that in his study at Shide, as he looks at the indications of
his instrument, he is able to say, for example, “Here is an earthquake,
and it is a little earthquake from Japan;” then a little later, when a
new earthquake begins, he will say, “And here is a big earthquake from
Trinidad.”
Professor Milne’s apparatus has brought us remarkable information with
regard to the interior of the earth. The story which we have to tell is
really one of the most astonishing in physical science. Let us suppose
that an earthquake originates in Japan. We shall assume that the
earthquake is a vigorous one, capable of producing bold and definite
indications on the seismometer even in the Isle of Wight. It is to be
noted that this instrument is not content merely with a single version
of the story of that earthquake; it will indeed repeat that story twice
more. First of all, about a quarter of an hour after a shock has taken
place in Japan, the pencil of the seismometer commences to record. But
this record, though quite distinct, is not so boldly indicated as the
subsequent records of the same event which will presently be received.
It is to be regarded as a precursor. After the first record is completed
there is a pause of perhaps three-quarters of an hour, and then the
pencil of the seismometer commences again. It commences to give an
earthquake record, but it is obviously only a second version of the same
earthquake. For the ups and downs traced by the pencil are just the same
relatively as before. The picture given of the earthquake is, however,
on a much larger scale than the one that is first sent. The extent of
the shaking of the instrument in this second record is greater than in
the first, and all the details are more boldly drawn.
After the second diagram has been received, there is yet another pause,
which may be perhaps for half an hour. Then, by the same pencil, a third
and last version is conveyed to the seismometer. This diagram is not
quite so strong as the last, though stronger than the first; in it
again, however, the faithful pencil tells, with many a detail, what
happened in this earthquake at Japan.
We have first to explain how it occurs that there are three versions of
the event, for it need hardly be said that the same earthquake did not
take place three different times over. The point is indeed a beautiful
one. The explanation is so astonishing that we should hardly credit it
were it not established upon evidence that does not admit of a moment’s
question.
[Illustration: Fig. 25.—EARTHQUAKE ROUTES FROM JAPAN TO THE ISLE OF
WIGHT.]
In the adjoining diagram we represent the position of Japan at one side
of the earth, and the Isle of Wight at the other. When the earthquake
takes place at Japan it originates, as we have said, a series of
vibrations through our globe. We must here distinguish between the
rocks—I might almost say the comparatively pliant rocks—which form the
earth’s crust, and those which form the intensely rigid core of the
interior of our globe. The vibrations which carry the tidings of the
earthquake spread through the rocks on the surface, from the centre of
the disturbance, in gradually enlarging circles. We may liken the spread
of these vibrations to the ripples in a pool of water which diverge from
the spot where a raindrop has fallen, or to the remarkable air-waves
from Krakatoa, to which we shall presently refer. The vibrations
transmitted by the rocks on the surface, or on the floor of the ocean,
will carry the message all over the earth. As these rocks are flexible,
at all events by comparison with the earth’s interior, the vibrations
will be correspondingly large, and will travel with vigour over land and
under sea. In due time they reach the Isle of Wight, where they set the
pencil of the seismometer at work. But there are different ways round
the earth from Japan to the Isle of Wight. There is the most direct
route across Asia and Europe; there is also the route across the
Pacific, America, and the Atlantic. The vibrations will travel by both
routes, and the former is the shorter of the two. The vibrations which
take the first route through the crust of the earth’s surface are
travelling by the shorter distance; they consequently reach Shide first,
and render their version of what has happened. But the vibrations which,
starting from the centre of the disturbance, move through the earth’s
crust in an opposite direction will also in their due course of
expansion reach the Isle of Wight. They will have had a longer journey,
and will consequently be somewhat enfeebled, though they will still
retain the characteristics marking the particular earthquake centre from
which they arose.
We thus account for both the second and the third of the different
versions of the earthquake which are received at Shide. And now for the
first of the three versions. This is the one which is of special
interest to us at present. The original subterranean impulse was, as we
have seen, propagated through the rocks forming the earth’s crust. Part
of it, however, entered into the core forming the earth’s interior. The
earthquake had the power not only of shaking the earth’s crust all over,
but it produced the astonishing effect of setting the whole interior of
our globe into a tremble. There was not a single particle of our earth,
from centre to surface, which was not made to vibrate, in some degree,
in consequence of the earthquake. Certain of these vibrations, spreading
from the centre of disturbance, took a direct course to the Isle of
Wight, right through the globe. They consequently had a shorter journey
in travelling from Tokio to Shide than those which went round the
earth’s crust. The former travelled near the chord, while the latter
travelled on the arc. Even for this reason alone the internal vibrations
might be expected to accomplish their journey more rapidly than the
superficial movements. With the same velocity they would take a shorter
time for the journey. There is, however, another reason for the lesser
time taken by the internal vibrations. Not only is the journey shorter,
but the speed with which these vibrations travel through the solid earth
is much greater than the speed with which superficial vibrations travel
through the crust. It has been shown that the average velocity of these
vibrations when travelling through the centre of the earth is rather
more than ten miles a second. The velocity varies with the square root
of the depth, and near the surface it is scarcely two miles a second.
There are two points to be specially noticed. The vibrations, which,
passing through the earth’s interior with a high velocity, arrive as
precursors, make a faithful diagram, but only on a very small scale. We
say that these vibrations have but small amplitude. This shows that the
particles in the earth’s interior are not much displaced by the
earthquake, as compared with those on the earth’s crust, and this is one
indication of the effective rigidity of the earth. It is also to be
noted that the great speed with which the vibrations traverse the solid
earth is a consequence of the extreme rigidity of our globe. These
vibrations travel more rapidly through the earth than they would do
through a bar of solid steel. In other words, we have here a proof that,
under the influence of the tremendous pressures characteristic of the
earth’s interior, the material of which that earth is composed,
notwithstanding the high temperature to which it is raised, possesses a
rigidity which is practically greater than that of steel itself.
[Illustration: SHOWING LOCALITIES OF EARTHQUAKES]
This is perhaps the most striking testimony that can be borne to the
rigidity of our globe; but we must not imagine that we are dependent
solely upon the phenomena of earthquakes for the demonstration of this
important point; there are other proofs. It can be shown that the ebb
and flow of the tides on our coasts would be very different from that
which they actually are were it not that the earth behaves as a rigid
globe. It has also been demonstrated that certain astronomical phenomena
connected with the way in which the earth turns round on its axis would
not be the same as we actually find them to be if the earth were not
solid in its interior.
The result of these investigations is to show that, though this globe of
ours must be excessively hot inside, so hot indeed that at ordinary
pressures even the most refractory solids would be liquefied or
vaporised, yet under the influence of the pressure to which its
materials are subjected the behaviour of that globe is as that of the
most rigidly solid body.
Happily in this country we do not often experience earthquakes other
than delicate movements shown by the record of the seismometer. But
though most of us live our lives without ever having felt an earthquake
shock, yet earthquakes do sometimes make themselves felt in Great
Britain. The map we here give, which was drawn by Professor J. P.
O’Reilly, indicates the localities in England in which from time to time
earthquake shocks have been experienced.
The internal heat of the earth, derived from the primæval nebula, is in
no way more strikingly illustrated than by the phenomena of volcanoes.
We have shown in this chapter that there is no longer any reason to
believe that the earth is fluid in its interior. The evidence has proved
that, under the extraordinary pressure which prevails in the earth, the
materials in the central portions of our globe behave with the
characteristics of solids rather than of liquids. But though this
applies to the deep-seated regions of our globe, it need not universally
apply at the surface or within a moderate depth from the surface. When
the circumstances are such that the pressure is relaxed, then the heat
is permitted to exercise its property of transforming the solids into
liquids. Masses of matter near the earth’s crust are thus, in certain
circumstances, and in certain localities, transformed into the fluid or
viscid form. In that state they may issue from a volcano and flow in
sluggish currents as lava.
There has been much difference of opinion as to the immediate cause of
volcanic action, but there can be little doubt that the energy which is
manifested in a volcanic eruption has been originally derived in some
way from the contraction of the primæval nebula. The extraordinary
vehemence that a volcanic eruption sometimes attains may be specially
illustrated by the case of the great eruption of Krakatoa. It is,
indeed, believed that in the annals of our earth there has been no
record of a volcanic eruption so vast as that which bears the name of
this little island in far Eastern seas, ten thousand miles from our
shores.
Until the year 1883 few had ever heard of Krakatoa. It was unknown to
fame, as are hundreds of other gems of glorious vegetation set in
tropical waters. It was not inhabited, but the natives from the
surrounding shores of Sumatra and Java used occasionally to draw their
canoes up on its beach, while they roamed through the jungle in search
of the wild fruits that there abounded. Geographers in early days hardly
condescended to notice Krakatoa; the name of the island on their maps
would have been far longer than the island itself. It was known to the
mariner who navigated the Straits of Sunda, for it was marked on his
charts as one of the perils of the intricate navigation in those waters.
It was no doubt recorded that the locality had been once, or more than
once, the seat of an active volcano. In fact, the island seemed to owe
its existence to some frightful eruption of bygone days; but for a
couple of centuries there had been no fresh outbreak. It almost seemed
as if Krakatoa might be regarded as a volcano that had become extinct.
In this respect it would only be like many other similar objects all
over the globe, or the countless extinct volcanoes all over the moon.
In 1883 Krakatoa suddenly sprang into notoriety. Insignificant though it
had hitherto seemed, the little island was soon to compel by its tones
of thunder the whole world to pay it instant attention. It was to become
the scene of a volcanic outbreak so appalling that it is destined to be
remembered throughout the ages. In the spring of that year there were
symptoms that the volcanic powers in Krakatoa were once more about to
awake from the slumber that had endured for many generations. Notable
warnings were given. Earthquakes were felt, and deep rumblings proceeded
from the earth, showing that some disturbance was in preparation, and
that the old volcano was again to burst forth after its long period of
rest. At first the eruption did not threaten to be of any serious type;
in fact, the good people of Batavia, so far from being terrified at what
was in progress in Krakatoa, thought the display was such an attraction
that they chartered a steamer and went forth for a pleasant picnic to
the island. Many of us, I am sure, would have been delighted to have
been able to join the party who were to witness so interesting a
spectacle. With cautious steps the more venturesome of the excursion
party clambered up the sides of the volcano, guided by the sounds which
were issuing from its summit. There they beheld a vast column of steam
pouring forth with terrific noise from a profound opening about thirty
yards in width.
As the summer of this dread year advanced the vigour of Krakatoa
steadily increased, the noises became more and more vehement; these were
presently audible on shores ten miles distant, and then twenty miles
distant; and still those noises waxed louder and louder, until the great
thunders of the volcano, now so rapidly developing, astonished the
inhabitants that dwelt over an area at least as large as Great Britain.
And there were other symptoms of the approaching catastrophe. With each
successive convulsion a quantity of fine dust was projected aloft into
the clouds. The wind could not carry this dust away as rapidly as it was
hurled upwards by Krakatoa, and accordingly the atmosphere became
heavily charged with suspended particles. A pall of darkness thus hung
over the adjoining seas and islands. Such was the thickness and the
density of these atmospheric volumes of Krakatoa dust that, for a
hundred miles around, the darkness of midnight prevailed at midday. Then
the awful tragedy of Krakatoa took place. Many thousands of the
unfortunate inhabitants of the adjacent shores of Sumatra and Java were
destined never to behold the sun again. They were presently swept away
to destruction in an invasion of the shore by the tremendous waves with
which the seas surrounding Krakatoa were agitated.
Gradually the development of the volcanic energy proceeded, and
gradually the terror of the inhabitants of the surrounding coasts rose
to a climax. July had ended before the manifestations of Krakatoa had
attained their full violence. As the days of August passed by the spasms
of Krakatoa waxed more and more vehement. By the middle of that month
the panic was widespread, for the supreme catastrophe was at hand.
[Illustration: Fig. 26.—SHOWING COASTS INVADED BY THE GREAT SEA-WAVES
FROM KRAKATOA.
(_From the Royal Society’s Reports._)]
On the night of Sunday, August 26th, 1883, the blackness of the
dust-clouds, now much thicker than ever in the Straits of Sunda and
adjacent parts of Sumatra and Java, was only occasionally illumined by
lurid flashes from the volcano. The Krakatoan thunders were on the point
of attaining their complete development. At the town of Batavia, a
hundred miles distant, there was no quiet that night. The houses
trembled with the subterranean violence, and the windows rattled as if
heavy artillery were being discharged in the streets. And still these
efforts seemed to be only rehearsing for the supreme display. By ten
o’clock on the morning of Monday, August 27th, 1883, the rehearsals were
over and the performance began. An overture, consisting of two or three
introductory explosions, was succeeded by a frightful convulsion which
tore away a large part of the island of Krakatoa and scattered it to the
winds of heaven. In that final effort all records of previous explosions
on this earth were completely broken.
This supreme effort it was which produced the mightiest noise that, so
far as we can ascertain, has ever been heard on this globe. It must have
been indeed a loud noise which could travel from Krakatoa to Batavia and
preserve its vehemence over so great a distance; but we should form a
very inadequate conception of the energy of the eruption of Krakatoa if
we thought that its sounds were heard by those merely a hundred miles
off. This would be little indeed compared with what is recorded, on
testimony which it is impossible to doubt.
[Illustration: THE EARLY STAGE OF THE ERUPTION OF KRAKATOA.
(_From a Photograph taken on May 27th, 1883._)]
Westward from Krakatoa stretches the wide expanse of the Indian Ocean.
On the opposite side from the Straits of Sunda lies the island of
Rodriguez, the distance from Krakatoa being almost three thousand miles.
It has been proved by evidence which cannot be doubted that the booming
of the great volcano attracted the attention of an intelligent
coastguard on Rodriguez, who carefully noted the character of the sounds
and the time of their occurrence. He had heard them just four hours
after the actual explosion, for this is the time the sound occupied on
its journey.
We shall better realise the extraordinary vehemence of this tremendous
noise if we imagine a similar event to take place in localities more
known to most of us than are the far Eastern seas.
If Vesuvius were vigorous enough to thunder forth like Krakatoa, how
great would be the consternation of the world! Such a report might be
heard by King Edward at Windsor, and by the Czar of all the Russias at
Moscow. It would astonish the German Emperor and all his subjects. It
would penetrate to the seclusion of the Sultan at Constantinople. Nansen
would still have been within its reach when he was furthest north, near
the Pole. It would have extended to the sources of the Nile, near the
Equator. It would have been heard by Mohammedan pilgrims at Mecca. It
would have reached the ears of exiles in Siberia. No inhabitant of
Persia would have been beyond its range, while passengers on half the
liners crossing the Atlantic would also catch the mighty reverberation.
The subject is of such exceptional interest that I may venture on
another illustration. Let us suppose that a similar earth-shaking event
took place in a central position in the United States. Let us say, for
example, that an explosion occurred at Pike’s Peak as resonant as that
from Krakatoa. It would certainly startle not a little the inhabitants
of Colorado far and wide. The ears of dwellers in the neighbouring
States would receive a considerable shock. With lessening intensity the
sound would spread much further around—indeed, it might be heard all
over the United States. The sonorous waves would roll over to the
Atlantic coast, they would be heard on the shores of the Pacific.
Florida would not be too far to the south, nor Alaska too remote to the
north. If, indeed, we could believe that the sound would travel as
freely over the great continent as it did across the Indian Ocean, then
we may boldly assert that every ear in North America might listen to the
thunder from Pike’s Peak, if it rivalled Krakatoa. The reverberation
might even be audible by skin-clad Eskimos amid the snows of Greenland,
and by naked Indians sweltering on the Orinoco. Can we doubt that
Krakatoa made the greatest noise that has ever been recorded?
[Illustration: Fig. 27.—SPREAD OF THE AIR-WAVE FROM KRAKATOA TO THE
ANTIPODES.
(_From the Royal Society’s Reports._)]
Among the many other incidents connected with this explosion, I may
specially mention the wonderful system of divergent ripples that started
in our atmosphere from the point at which the eruption took place. I
have called them ripples, from the obvious resemblance which they bear
to the circular expanding ripples produced by raindrops which fall upon
the still surface of water. But it would be more correct to say that
these objects were a series of great undulations which started from
Krakatoa and spread forth in ever-enlarging circles through our
atmosphere. The initial impetus was so tremendous that these waves
spread for hundreds and thousands of miles. They diverged, in fact,
until they put a mighty girdle round the earth, on a great circle of
which Krakatoa was the pole. The atmospheric waves, with the whole earth
now well in their grasp, advanced into the opposite hemisphere. In their
further progress they had necessarily to form gradually contracting
circles, until at last they converged to a point in Central America, at
the very opposite point of the diameter of our earth, eight thousand
miles from Krakatoa. Thus the waves completely embraced the earth. Every
part of our atmosphere had been set into a tingle by the great eruption.
In Great Britain the waves passed over our heads, the air in our
streets, the air in our houses, trembled from the volcanic impulse. The
very oxygen supplying our lungs was responding also to the supreme
convulsion which took place ten thousand miles away. It is needless to
object that this could not have taken place because we did not feel it.
Self-registering barometers have enabled these waves to be followed
unmistakably all over the globe.
Such was the energy with which these vibrations were initiated at
Krakatoa, that even when the waves thus arising had converged to the
point diametrically opposite in South America their vigour was not yet
exhausted. The waves were then, strange to say, reflected back from
their point of convergence to retrace their steps to Krakatoa. Starting
from Central America, they again described a series of enlarging
circles, until they embraced the whole earth. Then, advancing into the
opposite hemisphere, they gradually contracted until they had regained
the Straits of Sunda, from which they had set forth about thirty-six
hours previously. Here was, indeed, a unique experience. The air-waves
had twice gone from end to end of this globe of ours. Even then the
atmosphere did not subside until, after some more oscillations of
gradually fading intensity, at last they became evanescent.
But, besides these phenomenal undulations, this mighty incident at
Krakatoa has taught us other lessons on the constitution of our
atmosphere. We previously knew little, or I might almost say nothing, as
to the conditions prevailing above the height of ten miles overhead. We
were almost altogether ignorant of what the wind might be at an altitude
of, let us say, twenty miles. It was Krakatoa which first gave us a
little information which was greatly wanted. How could we learn what
winds were blowing at a height four times as great as the loftiest
mountain on the earth, and twice as great as the loftiest altitude to
which a balloon has ever soared? We could neither see these winds nor
feel them. How, then, could we learn whether they really existed? No
doubt a straw will show the way the wind blows, but there are no straws
up there. There was nothing to render the winds perceptible until
Krakatoa came to our aid. Krakatoa drove into those winds prodigious
quantities of dust. Hundreds of cubic miles of air were thus deprived of
that invisibility which they had hitherto maintained. They were thus
compelled to disclose those movements about which, neither before nor
since, have we had any opportunity of learning.
With eyes full of astonishment men watched those vast volumes of
Krakatoa dust start on a tremendous journey. Westward the dust of
Krakatoa took its way. Of course, everyone knows the so-called
tradewinds on our earth’s surface, which blow steadily in fixed
directions, and which are of such service to the mariner. But there is
yet another constant wind. We cannot call it a trade-wind, for it never
has rendered, and never will render, any service to navigation. It was
first disclosed by Krakatoa. Before the occurrence of that eruption no
one had the slightest suspicion that far up aloft, twenty miles over our
heads, a mighty tempest is incessantly hurrying with a speed much
greater than that of the awful hurricane which once laid so large a part
of Calcutta on the ground, and slew so many of its inhabitants.
Fortunately for humanity, this new trade-wind does not come within less
than twenty miles of the earth’s surface. We are thus preserved from the
fearful destruction that its unintermittent blasts would produce, blasts
against which no tree could stand, and which would, in ten minutes, do
as much damage to a city as would the most violent earthquake. When this
great wind had become charged with the dust of Krakatoa, then, for the
first and, I may add, for the only time, it stood revealed to human
vision. Then it was seen that this wind circled round the earth in the
vicinity of the Equator, and completed its circuit in about thirteen
days.
Please observe the contrast between this wind of which we are now
speaking and the waves to which we have just referred. The waves were
merely undulations or vibrations produced by the blow which our
atmosphere received from the explosion of Krakatoa, and these waves were
propagated through the atmosphere much in the same way as sound waves
are propagated. Indeed, these waves moved with the same velocity as
sound. But the current of air of which we are now speaking was not
produced by Krakatoa; it existed from all time, before Krakatoa was ever
heard of, and it exists at the present moment, and will doubtless exist
as long as the earth’s meteorological arrangements remain as they are at
present. All that Krakatoa did was simply to provide the charges of dust
by which for one brief period this wind was made visible.
In the autumn of 1883 the newspapers were full of accounts of strange
appearances in the heavens. The letters containing these accounts poured
in upon us from residents in Ceylon; they came from residents in the
West Indies, and from other tropical places. All had the same tale to
tell. Sometimes experienced observers assured us that the sun looked
blue; sometimes we were told of the amazement with which people beheld
the moon draped in vivid green. Other accounts told of curious halos,
and, in short, of the signs in the sun, the moon, and the stars, which
were exceedingly unusual, even if we do not say that they were
absolutely unprecedented.
Those who wrote to tell of the strange hues that the sun manifested to
travellers in Ceylon, or to planters in Jamaica, never dreamt of
attributing the phenomena to Krakatoa, many thousands of miles away. In
fact, these observers knew nothing at the time of the Krakatoa eruption,
and probably few of them, if any, had ever heard that such a place
existed. It was only gradually that the belief grew that these,
phenomena were due to Krakatoa. But when the accounts were carefully
compared, and when the dates were studied at which the phenomena were
witnessed in the various localities, it was demonstrated that these
phenomena, notwithstanding their worldwide distribution, had certainly
arisen from the eruption in this little island in the Straits of Sunda.
It was most assuredly Krakatoa that painted the sun and the moon, and
produced the other strange and weird phenomena in the tropics.
After a little time we learned what had actually happened. The dust
manufactured by the supreme convulsion was whirled round the earth in
the mighty atmospheric current into which the volcano discharged it. As
the dust-cloud was swept along by this incomparable hurricane, it showed
its presence in the most glorious manner by decking the sun and the moon
in hues of unaccustomed splendour and beauty. The blue colour in the sky
under ordinary circumstances is due to particles in the air, and when
the ordinary motes of the sunbeam were reinforced by the introduction of
the myriads of motes produced by Krakatoa, even the sun itself sometimes
showed a blue tint. Thus the progress of the great dust-cloud was traced
out by the extraordinary sky effects it produced, and from the progress
of the dust-cloud we inferred the movements of the invisible air current
which carried it along. Nor need it be thought that the quantity of
material projected from Krakatoa should have been inadequate to produce
effects of this worldwide description. Imagine that the material which
was blown to the winds of heaven by the supreme convulsion of Krakatoa
could be all recovered and swept into one vast heap. Imagine that the
heap were to have its bulk measured by a vessel consisting of a cube one
mile long, one mile broad, and one mile deep; it has been estimated that
even this prodigious vessel would have to be filled to the brim at least
ten times before all the products of Krakatoa had been measured.
It was in the late autumn of 1883 that the marvellous series of
celestial phenomena connected with the great eruption began to be
displayed in Great Britain. Then it was that the glory of the ordinary
sunsets was enhanced by a splendour which has dwelt in the memory of all
those who were permitted to see them. The frontispiece of this volume
contains a view of the sunset as seen at Chelsea at 4.40 p.m. on
November 26th, 1883. The picture was painted from nature by Mr. W.
Ascroft, and is given in the great work on Krakatoa which was published
by the Royal Society. There is not the least doubt that it was the dust
from Krakatoa which produced the beauty of those sunsets, and so long as
that dust remained suspended in our atmosphere, so long were strange
signs to be witnessed in the heavenly bodies. But the dust which had
been borne with unparalleled violence from the interior of the volcano,
the dust which had been shot aloft by the vehemence of the eruption to
an altitude of twenty miles, the dust which had thus been whirled round
and round our earth for perhaps a dozen times or more in this air
current, which carried it round in less than a fortnight, was endowed
with no power to resist for ever the law of gravitation which bids it
fall to the earth. It therefore gradually sank downwards. Owing,
however, to the great height to which it had been driven, owing to the
impetuous nature of the current by which it was hurried along, and owing
to the exceedingly minute particles of which it was composed, the act of
sinking was greatly protracted. Not until two years after the original
explosion had all the particles with which the air was charged by the
great eruption finally subsided on the earth.
At first there were some who refused to believe that the glory of the
sunsets in London could possibly be due to a volcano in the Straits of
Sunda, at a distance from England which was but little short of that of
Australia. But the gorgeous phenomena in England were found to be
simultaneous with like phenomena in other places all round the earth.
Once again the comparison of dates and other circumstances proved that
Krakatoa was the cause of these exceptional and most interesting
appearances.
Nor was the incident without a historical parallel, for has not Tennyson
told us of the call to St. Telemachus—
“Had the fierce ashes of some fiery peak
Been hurl’d so high they ranged about the globe?
For day by day, thro’ many a blood-red eve,
In that four-hundredth summer after Christ,
The wrathful sunset glared....”
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